CRF150 engine mods

[highmileage]

I wish to continue to use the OEM pistons in my CRF150 motard as I can use the next oversized OEM piston to legally stay under class max displacement, so the question is:

Is there room to mill the head to increase the stock 9.0:1 compression to something closer to 10.5:1 or 11:1. If so, how many thousandths can be removed without compromising cam chain length, etc?

Thanks...