Many of the Indian engines use a 20mm bolt as a plug, with string wrapped around it for sealing. The thread is very close to our 1/2" npt threads except that npt threads are tapered. I've had good success using a 1/2" pipe nipple and a ball valve. Use hard setting permatex to ensure a good seal. The permatex is the best sealer I've found. Teflon tape, pipe dope, teflon paste, they all ooze and eventually leak. Permatex has never let me down.