i was wondering if a bike this age can be fitted with more modern splined or even external bb,s , anyone tried this . trying to think of possible issues
They will work fine although you may have to swap one (and only one) of the spacers over to run external BB. I ran a 1993 Saracen Killi with Deore LX hollowtech2 cranks no bother at all. Chainline wasn't brilliant but acceptable. Octalink might be you best bet other that square taper (DO NOT get ISIS! they are an appalingly bad design!). Narrows your choice almost exclusively to Shimano cranks, but that isn't such a bad thing, and the BBs last ages.
As for getting the crank off, I'm guessing the cranks can be sacrificed yes? If it is a diamond drive crank then you can easily cut down the backward facing (away from the pedal) corner of the taper then hammer a sacrificial (or just old) screwdriver in the gap to open it out. If its a standard square you can try the same but it tends to be a bit more difficult.
The other, more 'proper' more engineeringy way to do it was to find a 2-pronged puller from somewhere. This is a device with 2 hooked legs that fit under the back face of the crank and a bolt that pushes against the BB axle.
Personally I have often used the "Me caveman me use hammer and saw" approach to it, usually successfully.