Many have been done, Think there were a couple at the thumpers today.There were certainly several with the indian style frame.
It depends how far you want to go and how serious a bike you want at the end. To get a ride-able bike lose all the tinware, change the top fork yoke for a trials style one, rebuild a 21 inch front wheel and make front mudguard brackets. Make up a sump shield (the one from hitchcocks is no good for trials.) change petrol tank for your favoured slimline tank, make up seat , make up rear mudguard brackets and make brackets to move footrests to a sensible position. Change the gearing in the engine (both primary and gearbox sprocket and rear sprocket (this involves work as they are cast onto brake drum ! (hitchcocks do a drum modified to take a different sprocket) , Change the ignition to a British mag and remove the advance and retard unit (may mean modding engine casing a bit) and if budget allows change the clutch housing for the earlier slimmer versions with no alternator inside. make up any bits not mentioned and i think you are there.
Don't think there is a real problem with the frames (could be wrong) as such as they original 1950's bikes were alwats good in rough conditions and indeed why the Indian army originally choose them in the first place.
If you want more serious get a crusdader type frame (they are slimmer and lighter) an early gearbox (slimmer) and go from there with improved fork internals etc). Also use early rear wheel (lighter) and machine front wheel to lighten.
My personal Enfield has most of this setup and is very competitive (i am not but the bike can be)
(crusader type frame, slim cases and box and rear wheel, std crusader forks with Indian internals, Indian alloy barrell and Indian 5 plate clutch)
If you serious about this route get to an EFA (eastern fourstroke association ) trial (mainly in suffolk and north essex) where there are a good half dozen in use and regularly wining events.