Aye tevie, might I suggest a look see and contact with this outfit in USA?
Honda_XL250S_big_bore_kitsAnother contact in the UK would be Eastwood Racing, Swanley, Kent, whom are importers of Wiseco pistons and Megacycle performance parts (cams) etc. I have dealt with Eastwood's and they know the Honda 4stroke and 2 stroke ranges very well. Vic the founder, having raced in the 1960's BBC Grandstand scrambles, with Jeff Smith et al.😊
If I remember, there was a write up and test of this mod using JE pistons in either the 'Cycle' or 'Dirt Bike' magazine's from USA, when this model was popular. Due to being crocked at the moment 😕 I cannot go and dig out the articles in my workshop that I think I have a copy of, once up on my feet I will see what I can find, but that could be some weeks away yet.
I hope this helps and please keep us posted of positive or negative developments.
My regards, Bill