I took the pedals off an 80’s Mercian which was VERY difficult. One came off eventually but the other came out with half the aluminium thread from the crank still attached. So now I need a new crank.
Can I fit any modern three ring square taper chainset or will there be problems if I use a nine speed set-up with my old 5 speed bike?

I might take this opportunity go go single speed . . .

“Should” be Ok I think – check clearance for the granny ring though.

You may have issues wuth chain width (and check it’s not worn out)

If it’s a LH crank you could replace that alone?

I have changed from a 70s cottered set-up to a modern square taper without any bother. I doubt 9-speed rings will love a 5-speed chain particularly though.

You could get your crank recoiled i.e. a new thread put in, just had it done by a bike shop and it cost me £14.

It’s a right hand crank unfortunately.

It’s square taper already but cranks is 86mm BCD so finding a single crank is unlikely.

If I have to replace chainrings will that mean replacing chain and cogs? What about derailleur?

You can still get a five or six speed screw-on cassettes – unfortunately they won’t be very nice ones but they will do the trick. Derailleur should be OK.

Mefty – Great idea. My LBS has just agreed to do this for £15

Great just make sure you put some anti seize (i use copper grease from halfords, i am sure there are fancier concoctions, but it has served me well) on when putting your pedals back on