Mark, rebuild the T-20 that you have. There are videos on Youtube that show step by step and Richard has instruction on here as well. It's not that hard to rebuild but some things take a little finesse and fiddling with. Check all your parts before you begin (case halves, drum faces, teeth, splines, bearings, shaft etc). Richard has most if not all the parts you may need including rebuild kits and bands. The only way I would look at getting a different T-20 is if you need more in parts than a complete T-20 would cost, if you can find one. After rebuilding it you will have a greater understanding of how it works.
Since you are rebuilding the Max IV, you may want to move the T-20 forward under the rear seat and lower the engine in the unit. WFO will also suggest making your T-20 a COOL T-20. This sounds like a complete rebuild so make all your modifications now for a better machine. Read through the posts on here to see what others have done to improve their Max IV's (Whipper and WFO's are a good place to start).
