I would say you're in for a T-20 rebuild. No telling what exactly is going on in there until you get it apart. Bands can be relined approximately $140, new bearings approximately $50 apiece (need to get these from Recreative industries). The thing that gets really expensive are the drums and sun gears, but chances are they are OK. Look over the T-20 build in the How to section and "crack" it open. Don't worry, there are a ton of people on the board that can help if you run into trouble. After the trans is good, get a new belt and check the Clutches out good. The secondary on the T-20 has a C clip on the backside that can break and cause the loss of power as you describe. The 18 HP is not bad if the clutching and the Trans is working as it should. I'd start there and fix your way back to the engine.
Last edited by micmac; 10-26-2011 at 08:19 PM.
"Don't worry my Dad's a TV repairman, he has an excellent set of tools..I can fix It"