Both of my Model A's have Model B motors. I went through 4 Model B motors to find two motors with counterbalanced crankshafts, motors without cracks, motors that had some shims left in the mains and that had standard size cylinder bores.
The most I have paid for a complete Model B motor is $300. The last Model B motor I bought cost me $125.
I always assume that any motor I buy will require professional rebuilding, even if I have a chance to drive it.
A Model B transmission is not a direct drop-in in a Model A, so a Model B transmission has no significant value in my opinion.