My cousins had one growing up on the farm that we played with, there were RV90's with a dual range transmission High and Low with 4 gears in each. The RV125 is just the 4 gears. About 8hp and 2-stroke. Tons of fun AND street legal. My brothers bought one for nostalgia and I've had fun helping fix it up. It'll do about 50mph with me on it. In the US they were called Trackers. In Japan they were called VanVan's, which means "Go"? Later became 4 stroke and might have even come back to the US, but I have yet to see a US 4 stroke VanVan.
Kinda like a Honda FatCat or a Yamaha BW200, neither of which is street legal I think. Tires are hard to get and expensive, some close fits in ATV tires but not really. In the end we just paid out the nose for OEMS that we found online.
If anyone has spare parts, I'll pass your info onto my brother, he's restoring the bike now and really has it sorted out very well.