I may be biased, but I've built the 77" Ziroli and put a ST3000 and 2 1/2 lbs of lead on the firewall, the smalles Stearman I have is 52" and I put a K&B .61 with a tuned pipe on it - no additional weight, the 67" Sterling's I've built need something like a .78 Fox to balance without any lead. When I build another 1/4 scale, I'll go with something like a G-38 or 42 to balance it without additional weight. There is SOME room in the front of the fuse for additional fuel tanks or smoke oil tanks, but that'll change the CG as its used.

If you want an inexpensive kit, try and find a Sterling (any of the 3 versions I know about). They take enough time to build that you'll probably be an adequate flyer by the time its done, they're relatively inexpensive (figure $200 plus engine by the time you're done), and they're robust IF you don't hit sideways on the nose. Forget the maple engine mounts and use a standard firewall mount.

Another thing to do it to replace heavy balsa in the tail with a better grade, sand everything down to minimum, use the tail wires to support it. Another thing is the real Stearman had 3 degrees positive incidence on the tail, 3 degrees postivie on the lower wing (I use zero degrees here) and 4 degrees positive on the upper wing. As you can see, the plane is supposed to fly on the wings and tail and not be dragged up by the engine.