Youd basically be stroking the 750 4.5mm as the 800 has a 66mm crank and the 750 a 61.5mm. If you are set on doing this put the crank in the cases and put the cylinders on and measure the deck height to see it the piston comes abve the cylinder top. If it does you could have the tops of the pistons machined where interference would be. You would also want to measure th squish to see if any valve to piston clearance issues have manifested themselves.
I dont have a 800 piston out so I cant say for sure the top is any different.
I dont have a 800 piston out so I cant say for sure the top is any different.