Cometic in the US will make a head gasket with any bore you like. I'm afraid the only way to be certain is to pull the head and measure the bore, most of the 610 kits had one piece scrapper rings which do let oil past, it is possible to modify the piston grooves, or rather have them modified, to accept 750 four standard size rings and fit the 3 piece ring.
You need to run this for a while and find out if the black is oil or fuel related before going farther