Flat slide taps can be pulled apart then mixed & matched as the internal threads on the upper & lower pieces seems to be the same regardless of the external thread with can be 1/8" , 3/16" or 1/4"
And these can be either BSPP ( 19 tpi) or NPPT ( 18 tpi) .
As to new bodies, they are all trash
The actual bodies are shorter and the hex is smaller
To cut costs the entire tap is assembled then barrel plated so the slide is plated on the outer end & bare in the middle
Thus when done up tight enough not to leak when off you can't push the slide through to the on position
The hex being smaller & the flat being wider means that the slide tends to chop out the body then become loose & leak.
Add to that the fact that the side is only partially chromed it chops up the corks every time it is turned on or off.
Then there is the splaying of the end of the slide to hold button on which is generally badly done so the buttons tend to wear their slit then fall off.

Now if you can find an original well worth cleaning and recorking but as for modern repos. hold onto them for long enough to make a profit reselling them for scrap or leave them on ebay where they belong, and that is definatly not on a functioning motorcycle .