FORDBOY -
Just so we have a reference here -
I ordered up a MGC tappet. We'll check it for hardness, but it is substantially larger in diameter than the standard A-series and B-series tappets.
If it can be made to fit, the world of camshaft lobe choices opens up considerably . . .

.9365-.9375 OD - about .001 taper bottom to top.
.809 ID toward the top, but it's stepped - goes down to .798 at .439 from the top.
1.554 from the top to the inside bottom.
1.750 overall height.
It does have an oil drain hole.
59 grams - ouch . . .