If your heads need 1/2" or 5/8" plugs, then one method would be to make some thick copper or brass washers as Terry mentioned and tune the height of some .750 reach plugs to fit your requirements. If you can get away with a .500 reach plug, then there might be an option there as well (see below) and you may not need special spacers.
Also, I don't know what your ignition and wires are. I tend to run solid core wires and magnetos (or high-energy ignitions) and do not use resistor plugs (I want as much spark energy as possible). I never run resistor wires/plugs with ignitions like a Harman-Collins dual-point as the yellow mid-plate will tend to leak secondary voltage all over the place when you put any resistance into the secondary circuit.
19 mm Reach (.750): I'd probably look at an NGK B6ES-11. This is a non-projected tip, heat-range of 6
12.7 mm Reach (.500): I'd probably look at an NGK B6HS. This is a non-projected tip, heat-range of 6
Links:
B6ES-11: NGK 7910
https://www.summitracing.com/parts/ngk-7910
B6HS: NGK 4510
https://www.amazon.com/NGK-Spark-Plu.../dp/B0053EZ6MK