Searching websites again.
Suzuki RM125 brake caliper doesn't look like it'd bolt onto the RM80 fork's caliper bracket.
Therefore, the RM125's rotor more-than-likely will not slip into the RM80's caliper. It MIGHT, but I don't know that.
The simplest option would be to try to instal the entire RM125 front end onto my bike. That'd be its fork, 21" wheel, tire, axle, rotor, caliper, hose and brake lever. If I'm lucky, the bike's handlebar clamps on. If I'm REALLY fortunate, the 1986 Suzuki 125 fork's triple tree slips right into my bike's frame. Okay, maybe after the correct bearings/races are installed.