The 125 has a smaller frame than the 250. This makes 250 tank too close to motor and too long to fit between radiators and seat-end. I adapted an IMS tank onto a '98 RM125, and it was a TON of work! Radiators moved forward, tank shimmed UP to clear pipe, Yamaha ATV petcock, melting/re-shaping of rear where sits against shock, moving kick-starter one notch rearward to clear tank, lower tank mount moved forward on frame... It can be done but patience and cleverness is the key. I'm sure similar tricks need to be performed with Clarke's tank. No-one advertizes one as fitting (and they'll tell you that on the phone too).