Hi David,
the taper on these taps is anything of a taper, not a standardised type I guess. So you´d clamp the brass item in the lathe somehow and set the lathe up for this angle with an indicator on the taper running along the taper. So you should be close enough for a try with the petcock body and some minimal corrections possibly on the lathe when you fabricate a stainless steel copy. Some lapping with grinding paste will be essential for a seal but it is worthwhile, no more troubles with any fuel that attacks most material except Viton or teflon.
Vic