I gave it a good two-hour search, but I can't find it. It may be defunct. On the other hand, googling "SR500" is like googling "Bob Smith". If anyone knows the company, they were low-countries based and specialized in cafe racer and retro bits for Yamaha singles. I think they were the first ones to turn out an SR-engined BSA Goldstar replica. I had an SR 250 at the time, hence my interest.
During my searches today, however, I note that quite a few people have retrofitted Harley springer and Brit girders to their Yamahas with apparently little effort. Don't see why it can't be done with the RE, too.