Hi Gerry, I don’t think any of the self contained starters will fit down into the seat far enough .I got my starter from TS racing,then a couple of heavy duty cables about 6′ long and good marine /RV batt and your set.
This guy’s probably got the nut you need. I got one for a cr125 (same as rs125)…call him to verify fit for RS250/CR250?
Uses LH thread, goes in flywheel like a puller.
His “drill setup” seems a bit pricey, and may be too long for your seat as well?
I use standard kart starter with separate battery, but I’m a F125 laydown so i’ve got more room.