Leningrad frees up 2 Finnish corps. it doesn't change the fact that the Murmansk railroad is hundreds of kilometers away from Finnish railroads-how are the Finns going to supply a drive across the frozen wilderness against a USSR that can ship in troops and supplies by the trainload? The only railroad at the time in Northern Finland ran along the Baltic, the closest railroad to the Murmansk line was Joensuu to Sortavala, still 200 kilometers within from the Soviet border.
tl;dr The Soviets did not have troops and supplies by the trainloads at the time when the Finns historically contemplated, prepared for, and then called off the offensive against Sorokka. The RKKA forces holding the Sorokka (Belomorsk) front were tired and understrength after the summer months. The reasons to cancel the offensive plans were mainly political (Germany had failed to seize Moscow and Japan had brought United States to the war).
Now, the long story:
By February 1942 the Finnish state railways were already supplying the forces in occupied Eastern Karelia with rail connection up to Karhumäki (Medvezhyegorsk). Thus the distance they would need to cover from the nearest railhead was nothing these same forces had not done before in similar terrain and force ratios during the invasion and occupation of Eastern Karelia.
This is how they planned to proceed with the offensive historically:
Mannerheim negotiated about the potential for new operations with A.O.K. Norwegen's new commander-in-chief, General Edvard Dietl, at the beginning of February 1942, after the Germans once again tried to persuade Mannerheim to attack. Dietl promised Luftwaffe support for potential future operations. However, in the opinion of both, there were no opportunities for the operation at the time being
because the existing forces in the region were tied down to other tasks. Due to the winter conditions, the German attack towards Kantalahti also had to be abandoned. According to Dietl, the Germans aimed to have enough troops available by the summer to carry out the operation. Mannerheim announced Keitel at the beginning of February that there were no conditions for the operation to reach Sorokka. General Airo, however, was tasked with drawing up a plan of attack.
According to Lieutenant General Wiljo (W.E.) Tuompo, Head of Headquarters Command Staff, at the beginning of February Mannerheim considered the possibility of an offensive against Kotskoma. Previously, he didn't want the operation to be mentioned due political considerations, but after the US had broken diplomatic relations with Finland, he seemed to have reconsidered his options. The potential operation was planned to be carried out during the winter, since such conditions were deemed favourable for Finns. Both General Airo and Tuompo turned out to be supporters of the operation.
At the beginning of March 1942, Airo left a memo according to which the capture and holding of Soroka could not be carried out
with the available forces. The Finnish HQ was aware of the appearance of fresh Soviet divisions in the isthmus between Seesjärvi and Äänisjärvi and their activity. According to the memo, the Äänisjärvi – Seesjärvi isthmus was the area through which the Finnish offensive would take place. At the end of the memo, it was stated that the winter operation against Sorokka in spring 1942 would have only a small chance of success by March. On the other hand, the forces in the Rukajärvi direction were deemed able to defeat the opposing lone division and reach Kotskoma. The planning of the operation and the already started preparations continued. Liasons were established with Luftlotte 5 for future ground support operations.
A Finnish intel report prepared on March 10, 1942 estimated the frontline strength of the Soviet defensive positions in the direction of Tunkua and Kotskoma has been as c. 8 300 soldier. In total, it has been estimated that there could be approx. 12,000 Soviet soldiers. The strength of the Finnish planned offensive was c. 27,300 men, in which case the calculated strength ratio would have been about 2.3:1 in favor of the Finns.
The Finnish HQ estimated that the maximum amount of forces that could have been supplied in the region were 44,821 soldiers if two extra divisions, which have not yet been deployed from general reserves, had been added to the aforementioned Finnish forces. In that case, the power ratio would have been about 3.7:1 in favor of the Finns. Historically the Finns were forced to use their new 8th Division to protect the Karhumäki region. The use of the 8th division could also be considered as a reserve for a possible attack from the area of the Maaselä group towards the Viena canal. In OTL the use of the division as a reserve or in attack was unlikely, because pulling it from the front line from the Maaselä isthmus would have hardly been possible. If the 4th Division had been left out of the calculations, the total strength of the Finns would have been 39,589 soldiers, leading to 3.3:1 force ratio.
The Finnish General Staff thought the operation as entirely feasible considering the weak state of the Soviet forces in the area in spring 1942. A fall of Leningrad might prompt Mannerheim to give it a go. Now, as you mentioned, the Soviets would definitively make the the defence of this region a priority, and would allocate forces here. But from where, which forces, and what effects this would have to the rest of the Eastern Front in winter 1942?