The kit that Megadan listed says it is for 150cc motors. I'm thinking that if it would fit a 125, they would have said so. Probably there is something that makes it not work in the 125. Such as the stud spacing.
It might save you a lot of hassle to get a base or head gasket for the 250 and see if the stud holes line up with your 125 studs. Or get someone to measure their CG250's stud distances so you can check that against yours. I fear that you are trying to put a "big block" cylinder on a "small block" case. I also suspect that the 125 crank was never intended to handle the 250 power.
Something like this would give you 150cc and be simple, cheap and designed to fit your motor:
https://www.aliexpress.us/item/22518...Cquery_from%3A
And a CG150 head:
https://www.aliexpress.us/item/32568...Cquery_from%3A