The RMX was based on the '89-'92 RM250. Lots of parts interchange. Changes were bigger 3-gal. gas tank (still slim, tho) 18" rear wheel, kick stand, case protectors, lighting coil & lights, heavier flywheel, slightly-milder porting, slightly-softenend suspension, odometer, double-walled pipe & spark arrester.
Most say all one needs to do to get RM-like HP figures is to replace the exhaust & install a '89 RM250 head gasket. I replace these on mine before I ever started it up, so I can only take other's words for it; it will not win drag races against MX bikes, but once clicked into 5th, can leave them behind, & it much easier to handle in the rocks where throttle control is required with the engine & flywheel weight differences.
The '93-'95 RMs were updated somewhat, whereas the RMX received fewer changes; likewise in '96-'98 when the RMX got conventional forks, slightly heavier springs, & some more HP. In '99 Suzuki stopped importing the RMX (EPA/CARB requirements), but the rest of the world got a new one based on the '96-'97 RM; slim but with all the fine enduro features mentioned above.
There are better off-road specific 2-strk bikes today (Gas Gas, KTM, Husky) but none come from Japan. I plan on never selling mine.
Minuses: They are heavier than MX bikes (still lighter than the 4 strokes), & some like to rag on yellow bikes. Oh: the stock (MX!) carb is difficult to jet for idle/off-of idle. I replaced it with a better PWK carb.
Pluses: Very reliable, dependable, cheap to keep; a bargain these days. Absolutely excellent suspension setup for off road. Easy to ride, turns well. Good but not overwhelming power. Can still get good-looking graphics kits.