MVVS does not suggest other than the Brisk UR17, to which the caps are adapted. This is a standard tapered seat garden appliance plug, and in MVVS engines they are excellent.
Champion DJ7Y (long nose) and DJ6J (cold) have similar dimensions, but I have not had opportunity to check out the R-types,
Bosch HS8E (hot) or HS5E (cold) may also fit.
NGK BM5F, 6F and 7F (hot>>>cold) may be candidates if the internal spring can compensate for dimensional differences.
MVVS still supplies the UR17 plugs, so Horizon will be able to supply them. I do as well.