I think the current Yak-1b fits well the Kuban scenario becuase from what I read here it's an early 1943 variant. They could have chosen one of the initial variants (October 1942?) which wouldn't have some of the aerodynamic improvements but at the same time would fit in the late Stalingrad campaign.
About the P-39L, I don't think it's one of the worse variants, you have other more numerous earlier ones like the Airacobra mk I (P-400) without armor and self-sealing fuel tanks, or the first main variant the P-39D (which had a worse engine?) The L version is basically a K with a different propeller (Don't know if better or worse though). Someone posted here that the P-39L was the most numerous at the Battle of Kuban so for me it's ok.
Use of Airacobra Is (some 150-160 received by Soviets) mostly happened in the North. At the outset of Kuban battle the first regiments to fly the Cobra flew the P-39D-2 and P-39K models, with some P-39Ls also.
Differences between P-39D-2, K and L are minor, with the L being slightly heavier. P-39D-2 actually has the same engine as -K and -L versions.
When I say most numerous - well. Soviets received 158 Airacobra Is from the British, and from the US, they got 108 P-39Ds (mostly D-2), 40 P-39Ks, 137 P-39Ls, 157 P-39Ms, 1113 P-39Ns and 3041 P-39Qs.
As for what was most numerous in Kuban, well, there I don't know for sure. I know at the outset of the Kuban battle the first regiments to fly the Cobra flew a mix of P-39D-2s, P-39Ks and P-39Ls. Introduction of P-39M/N in some regiments started in May 1943. By the end of the battle of Kuban, they were by all indication common.
Both Soviet and American P-39D-2 manuals are interesting to read; from them we can surmise that a significant number of aircraft were apparently refit with automatic boost control, which is not the case with P-39K/L manual, and which would for purposes of the game be a very significant difference.
About the Yak... they chose the Yak-7b which is a sidegrade compared to the Yak-1 (more guns yet more weight and less agility I would expect) and a downgrade compared to the Yak-1b. They could have made a Yak-9 which would have been comparable to the 1b.
True, but out of possible Yaks, I would rate the -1b to be the best for our purposes.
I have to agree with the poster above that the Yak is somewhat optimistic, also.