Head Measuring

The bore size should be identical to the theoretical standard bore size +.020" in your case.  The necessary clearances are built in to the piston size by the manufacturer. I have some new Heppolites and the instructions in the box are perfectly clear on this.  Bores are best measured by a proper bore gauge, like a Mercer gauge. Mine is first calibrated to the correct size using a large micrometer which has its own standard calibration rods. The Mercer gauge dial is zeroed, and any departures can be measured to .0005" (half a thou).

Other methods are no doubt available.

Once you have accurately measured the wear, how much wear will you accept?. My 99 had been running and in regular use with a bore that needed all of 10 thou to clean up. The ring gaps must have been huge.  As you have all the parts I would be tempted to get the top ridge removed and a light hone and assemble , What is there to loose?.

Exactly!,  But seriously  unless you have a considerable ridge at the top or  deep scoring of the bore, you have a fair chance of some years of classic use.  You could scrap the parts spend £200 + getting it all like new ,only for it to seize  solid  while running in . Then you will be going for the last rebore your motor can stand.  Its most unlikely to seize with the existing parts even if the timing and carb are not perfect  (and thats more likely than not ).The motor may burn a little oil till run in and may not be as quiet as a new set up ,but thats old bikes. Put a new ring square  in the bore at the bottom and measure the gap,then compare with just below the ridge at the top,let us know what you find.