why not simply use a 1/2 BSP 1/4 turn azy packed plug cock?.
As far as I was aware thats essentially what yours is on 'The Pirate' Tryst, only later did they fit the push button type.
One with screwed fittings like this
£18.40 plus some very awfull tax at
HSS, probably with you the following day too.
Just need to make the pipework, and a handle and rod then your good to go