5 inch bore over 4.75 is only a further 6% displacement, so should not particularly overload a good starter.
However the starter will accept 12 volts with no real problem as long as not cranked excessively. Many have simply exchanged the 6 Volt battery for a 12 Volt.
The dynamo would need changing, of course.
Questions of your connections could be made. Wiring too, if it has a marginal cross section.
My 90 used to start almost always on 4th or 5th pull up of the handle. I agree, after a heart bypass, it can be a bit tiring to hand start - I fitted a starter to help me out.
With lighting you clearly need a generator fitted; but for starting only, if it has a magneto the generator could be left disconnected. All depends on how original you want it. Fitting bigger liners and pistons has already made a dent, I suppose.