Great flying model & worthy of a quality restoration! Your Joker likely used the MK beam type soft mount, you mat be able to find them in 60 size from Central Hobbies. They work fine on 60 size 2 strokes, personally I didn't have luck with MK mounts on 4 cycles.

What are you planning for power? Mine used an OS 61RF-P & it was plenty of go...YS 60-61 would be good too. I saw a few back in the day with Enya & OS 1.20 4 cycles- too heavy for their power (this was pre-YS times). Keep it 2 stroke, simple & light...and oh yeah..gotta have retracts!