Hello Dave : Well just about everything has been tried with a Centaur but the facts are there are just true limitations in the Centaurs drive train. I don't think the reverse gearbox would stand up to more power and the applied torque that a motorcycle clutch will put out. The torque converter as it was in the original Centaur still took out a lot of the gearboxes. A lot ran out of lubrication and shifting from forward to reverse while the input shaft was still spinning crunched a lot of gears ant that led to ground up inter parts. The failure of the cable disc brake and when it got out of adjustment and did not do it's job the gearbox suffered.
So any changes to a 4 stroke might suffer lots of issues. I went the VW way as did most street trikes.