Studs for exhaust valve covers

Which studs are you relating to?

Exhaust rocker covers stud thread in the cylinder head is 1/4 Whitworth

Hi Geoff - I presume you mean the exhaust rocker covers.  Into the head the studs are 1/4 inch BSW .  If you're lucky these helicoil insert kits are still available  (BSW is more commonly know as Whitworth thread). If you can't get the kit a 1/4 inch UNC is extremely close ( same tpi but slightly different angle of thread) . Good luck, Howard
   

have a look on RGM  site they sell studs and nuts 

helicoil kit 050731 part number
dome nuts 067552S
Studs 067582

If you are having to repair the thread whilst on the bike I'd go straight in with the tap. With it being aluminium I would want to avoid using a drill by hand in an awkward position.
In my experience a stripped thread in aluminium will leave the cavity ready for a helicoil hand tap without any bother.
Pre spray with WD set your tap square in line with the hole and keep an even pressure until you have the first couple of threads cut, feel your way to proceed to fully bottom out the tap.
When applying the coil don't forget to set the application wrench collar to the length of the coil plus the tang fork depth!
WD the tapped hole, then keep an even pressure on the tool as you progress the coil in further down as far as it wants to go.
The idea of the collar is to ensure that the coils follow each other and don't leave an empty thread, if this should happen just pull the banty thread out and try again with a new one.
 

What about just making a new over size stud? Much cheaper than buying helicoil sets, but of course you need a lathe and some taps and dies.

I used to buy mine from Uni-Thread.  Currently their 1/4" BSW Kit is £24.78.  A bit pricey for just one repair. Useful for the other 1/4" threads on the bike though.