Sparky,

those MK 20's are wonderful little classics!

3.5 lbs is right in line with spec weight - a little lighter on a smaller light engine like the K&B 21. Folks have put up to 32 and 35's in these MK 20's but mainly in the larger ones like the Aurora and Cosmos. Another excellent RE engine choice would be the NovaRossi R528 - a 28 size engine with outstanding performance.

For the pipe, just bolt a small cradle base (like that in the DB units or made from dowel and 1/32" ply) and hold the pipe to it with a good tie wrap. Keep it toward the rear of the pipe where the temps are lower. You can use heat resistant tape between the pipe and cradle. Avoid metal to metal contact.

Post some pictures!

David