Bajaj Classic 125 to 150

[JohnMcJet200]

Hi all,
Looking for a bit help.

I bought a Bajaj 125 Classic SL for a winter bike. It was dead slow and I could not upgrade the exhaust as it was a strange set up.

So I thought I would upgrade to a 150 with a SIP exhaust on it.

This is were the problems begin as I believe the 125 has a 54mm crank so the piston sat way above the top of the barrel.
So after finding a mention in a random tuning page it stated an 8mm spacer, wat to big. Would not pull a skin off a custard.
Dropped it to 6mm and is better.
I was thinking dropping to 4.5mm which should bring the piston in line with the top of the piston.

Has anyone done this conversion before?
Any helpful tips will be gratefully accepted.

Thanks
John