Manual Transmission - T56 sticking in gear

I have a close friend with an ls1 swapped rx7 and his t56 is sticking in gear. What possibly could be causing this and what route should we take on fixing the issue? New trans, take it to a tranny shop and have them diagnose it, or buy a rebuild kit and have a shop rebuild it? 600rwhp is in the future.

Broke a slider key for a synchro, common cause for sticking in gear issues.
Your basically tearing the trans down for a rebuild to fix this.
Billet slider keys are available for 3/4 assembly.