What a great mower - I had one for a few years. Yep, the self-propel is the weak point. I don't see why you couldn't put a transmission in it - plenty of help available here.

It is a challenging experience. There is a lot to do for "only" a transmission. Honda build nice mowers, but traditionally anything trans related seems ridiculous for a walk behind mower. If you possess average mechanical skills, snap pics and take good notes, you should be fine. Watch over the disassembly of the wheel clutches. Hopefully the adjuster arms come off ok. Might I add a new belt should be installed while you have it off.

Hmmm...sounds like fun. Thanks for the advice. Not sure I know exactly what a wheel clutch or an adjuster arm is, but I do have a digital camera so that's a start!

Are you sure it is the transmission? Did you check the belt?
The reason I ask is that I have many parts for the TDA and all the 3 speed transmissions have no problems.
I would make you a deal on a good used one, but the 3 speed is usually good.
The 2 speed and HXA have had problems, but the 3 speed is usually the one without problems.
The HXA usually has problems pulling backwards and the 2 speed usually skips.

I did not check the belt personally, but I took the mower in for a check when the drive quit working...they came back with an estimate for transmission replacement. I just picked up the mower and ran it without the tranny all summer. So to answer your question, I am trusting the mower shop on the transmission being bad...