Another option you might consider is an Air Trikes PSRU. They're in Canada and make gearboxes that look similar to Rotax for a lot of different engines. They are fairly well known in the trike world and he seems to have a good reputation for reliability, service and reasonable pricing. I have an NSI gearbox on a Subaru EJ25. NSI is no more, so if it ever fails I think I'd go the Air Trikes route.