which plug

[bull]

When doing a BBK upgrade, which spark plug to use is only 1 consideration. and should be the last based upon what you did before it

the following information is relevant to GY6 50cc (39mm) stock engines:

the stock 39mm piston has a convex dome that increases compression.
the stock 44mm piston has a convex dome that increases compression
the BBK upgrade 47mm, 50mm, & 52mm piston has a concave dome, reduces compression

heads; stock 39mm & 44mm have smaller areas, higher compression
47 to 52 mm have larger areas & bigger valves, lower compression

gaskets; many of the BBK suppliers supply gaskets that are thicker which reduces compression, and puts stress on the camshaft and journals; stress is extra friction which reduces power and harms the engine

done right a 47mm BBK can supply enough power to perform exceptionally well and in some cases outperform the 50 & 52

only 1 supplier I have found that offers enough product differences to allow customization and obtaining the most power and he has to source from several wholesalers to get it