I dont think it is possible to use the offset bore cups approach, as seen on the TL125. If they are like the RGV they are scooped/hollowed out on the backside for lightness, and the big end pins are too close to the edge of the crank.

If the integral crank pins are solid you could always offset grind them down and use a rod with a smaller big end ID and bearing, but that only gives you about 1mm on teh throw, and a very difficult spacer to make up for the hole on the other side. The integral pins could also be offset built up with weld for another 1mm on the throw, giving 4mm on the stroke.

But its a lot of hassle and Im not sure about reliability...

I am working on chopping up an 54mm stroke RD250 crank to fit an RGV250 at 50.6mm - could the same Idea apply here? There are several KTM/japanese 200/220cc strokers around 57-60mm stroke? Might be hard work but possible not as hard as modifying teh stock crank. But where would the fun be if it were easy....
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NSRless again...