TDR90 replacement
No cheap solution if you want to keep a CTL-style controller. You'd have TDR94 with CTL92E, but also a big bill!
Try a Becker BXP6401 transponder fitted in place of the CTL92 - that's all you need. Wire to existing antenna and altitude encoder. The transponder has a dongle (programming module) that plugs into an additional connector to store the Mode S address, so you don't have to re-programme if swapping a faulty unit. You should allow for squat switch input - JAA TGL13 for ELS is shortly going to be replaced with an new EASA AMC which will make this a mandatory requirement - either a squat switch or an airspeed switch for automatic GND mode operation.
We've done this Becker mod a A109's where CTL was mounted in the glareshield panel. It works very well.
Regards