stupid idea? xr250 cr125

So I don't know if this is a stupid idea but basically I have an old steel framed 1994 road legal cr125 with a set of road and off road tyres, the 125 engine was a good blast for green lanes whilst it worked but was pretty bad on the road as can be imagined, drank fuel and constantly lost water through the head gasket which I could never fix, the engine was pretty much shot and had been messed with before I got it and struggled with it for a long time.

Now the bike has been sat in the shed for the past 2 years and been looking around and wondering if its possible to squeeze something like an xr250 engine in it? as far as ive read online they're pretty bomb proof which is what im after, and hopefully a lot more efficient.

I'm not too fussed about power but have no idea whether it would be an easy swap or not, I can weld and have a fair few tools at my disposal so If anyone has any ideas? links? suggestions? that would be great help

Thanks!