A $20.00 set of dial calipers will be accurate enough to get a ballpark guesstimate of bore size. A set of telescoping gauges and/or an inside micrometer will be more accurate. Be sure to measure about 1/4" or more below the top of the block to avoid getting an incorrect measurement due to carbon buildup above the piston ring travel, or remove the carbon with a wire wheel in a drill motor before measuring. Also check the taper of the cylinder. If it's tapered much over .005" top to bottom, the block will need to be rebored.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!