If your friend has a C-85-12 he has a rear cover that will accept the starter & generator or alternator. If he has a C-85-8 there are no provisions to fit them and it will be expensive.
The problem with the Chief is that there is not enough clearance for the starter from the firewall and a modified engine mount must be used. A guy in South Dakota will supply one with an STC for $355. Buzz Wagner (605) 532-3862, don't forget the time change!!
You do not need to change the nosebowl as someone suggests although with a slightly longer engine mount slightly larger wrap around cowlings would be needed.
I once fitted a C-90-8 powered Super Cub. in UK on a full CofA, with a complete electrical system. It was full IFR and the 35amp battery had to be recharged after about 2 1/2hrs continuous use, which was rare, so it worked very well for mostly VFR flying.
I presume that the aircraft is on a Permit to Fly so a call to the PFA may find some other modification that would help.