engine has a tag!!!

To tell what size the old engine is - you either get the engine builders spec sheet from when he built it or you tear it apart and measure piston diameter and the crank throw.  The tag most likely came from engine builder and means something to only him. 

If you car is new - call the car builder (replica factory) and see if they keep records.  Who built the Speedster and when?

If the motor runs well, you can just pull one head, then using a couple spacers and nuts, hold the cylinders to the case. Now measure bore and stroke, further disassembly not needed.

I just did this on Friday on Chubby-Vinnie's motor  ..Pull the rocker arm assembly and remove the head bolts..carefully slide the head back to isolate from the cylinder jug,  clean the top of the piston and you'll see the the bore size is stamped on the top . Make sure you torque the head bolts back first to 10 lbs then 18 lbs and finally to specs which I believe is 28 lbs DO torque in the correct order that being from the inside center bolts out.

Would piston also maybe have an A or B on it which would help determining if stroker crank?  I assume all B pistons are for stroker cranks.  Is it as simple as rotating crank with head off (but jug bolted tight) and measuring throw with 69mm being stock or 74 or 78 being stroked?  That just leave cam in question.