What parts, if any, are compatible with a TRS drive that are on a TR drive? (i.e. lower housing, bearings, gears, prop shaft, etc.)

Upper gear housing is the same. Lower gear housing and everything in it are different. Here is a diagram where you can compare them.

http://www.perfprotech.com/store/cat...6022-4280.aspx

Thanks for the info. The lowers housings looked the same, but I didn't have the two side-by-side.

If you seen them side by side you would see that the TR is longer. You can run up to a 20" diameter prop on a TR where as you can only go up to a 15" on a TRS. TR has a 2:1 gear ratio, TRS has 1.5:1 ratio. The TR propeller shaft is also a larger diameter than the TRS. Basically the TRS is for pleasure and performance boats where as the TR is a slow power drive for large heavy boats like house boats and work boats.