With respect to the Transponder, Airwich in the USA has a huge inventory of used Cessna boat anchors.....er excuse me Cessna Avionics. The other option is a new slide in replacement unit like the NARCO AT155R. However one of the problems with Cessna radio packages is the poor quality factory installations. When my transponder stared acting up for the second time I replaced it with a Garmin GTX 320. I am extremely happy with it. There is no warm up it, draws about a quarter of the power, and is lighter.
With respect to the autopilot, it is vital that you provide a comprehensive list of what it is actually doing or not doing to a repair facility. For instance
1) Does it correctly self test on the ground? If yes wait 5t min with engines running and radio master on and repeat. Does it still self test ?
2) Start with aircraft in trimmed level flight with A/P off. Does the electric trim work smoothly and correctly ?
3) engage A/P with heading bug on current track with alt hold off. Does A/P follow heading bug? does the roll control knob work ?
4) engage alt hold. Does the aircraft immediately pitch up or down ? Does the pitch control wheel work ? Does it work smoothly and equally in both directions ?
5) If the A/P kicks off, how long before it stops working ? Can it be reengaged? Does all of it stop working or only part of its functions?
6) Does the nav tracking work ? will it intercept and track a nav course ?
Providing a clear description of the probem will save you big money at the Avionics shop.