As long as your system has two AC+ terminals and then the other going to battery or ammeter, then you can use a generic one. Charging system doesn't care what it looks like, just as long as it has the 2-AC and B+ terminals.
I had one similar to yours on my Sears with a Tecumseh, I switched over to these generic ones and it's fine. I did run a separate little ground wire form the mounted bolt just to make sure I had good ground and I had to cut off my factory terminals and crimp on spade connectors.
I do a search on ebay for an Onan voltage regulator, just make sure I get one rated for 20amps.