You can replace the trim's variable resistor with a miniature from RadioShack, Fry's, or the like. Remove the wire from one outside lug of the original, measure the resistance from one outside lug to the other, go to your local electronics parts supplier and get a replacement with a value close to what the original is (if you go SLIGHTLY higher, you may get more granular trim control).
Find an rea inside the xmitr box where you can drill a hole and mount the replacement where you can reach the shaft from the outside withe the xmitr box closed up.

It may look hokey, and you will have to readjust/check the trim each time you use the controller, but, it'll work.