Can anybody remember what piston to use to pop out a TL 125.

The piston to take it out to a 145cc is Honda CB750 (1969-1978) piston. The best idea is to get a Sammy Miller kit if they still do it. The kit contains the very thick head gasket that you will need or the compression will be horrendous. You can pile up base gaskets and cut your own head gasket (for the bigger hole) but that is a complicated thing to figure out.

Millers still list the kit so it shouldn't be too hard to get hold of

Thanks for the replies.