How much can be machined of the top of 73.5 hypertec Pistons eg 1.5 , 2 , 2.5 , 3mm this is for a1430 cc motor , thanks

The big issue is not what you take off the top but how much you can machine the dish to get the CR to a sane level. And then how close the top ring gets to the top of the bore.

Easiest way to see how much meat you have is to measure from the bottom of the skirt to the crown (ie the overall piston height) and then measure from the bottom of the skirt to the bottom of the crown casting inside the piston. If you're getting much thinner than 120 thou or so then I would get the crowns thermally coated and make sure you don't run into any sort of detonation.

A better way may be to get a set of Omega pistons from MED in the UK. They do a set of short crown height pistons for stroker motors. I ran a set in a 1460 I built up and they were excellent.

Both Hypatecs and Russell Engineering pistons have thick crowns, you can remove well over 1.5mm and still have plenty of thickness.
However, the Hypatecs have the top ring further down, I would prefer these if doing a stroker. As I have done a few times.
As for the bowl shape, the Hypatecs can have the dish machined to have a flat base and an M5 radius, as the Russell ones do.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Graham did my Hypatecs .10cc in the dish Lots of meat in em and space above top ring groove.