Recently was unable to loosen the bolts of an electric fan motor that needs rebuilding.

Have never had this problem before so my first thought is the bolts have a high strength thread locker.

Did a forum search to check if anyone else had mentioned this problem but found nothing.

Am interested in all suggestions to loosen these bolts.

As said some localised heat (gas torch) preceeded by a squirt of some
penetrant like wd40, penetrene etc and use a correct size socket or ring
spanner.

Hitting it with a gas-torch flame, after having sprayed it with a
penetrant which is made up of a high percentage of flammable petroleum
distillates?

That seems like a Really Bad Idea. Not quite as bad as the fabled
"Hey, Bud, I think my truck's gas tank is empty... hold my beer while
I light a match and see if I can see any gasoline down the filler
pipe" but it's headed out in that general direction.

I'd use either heat, or a penetrating oil, but not both. In the OP's
situation, since a high-strength threadlocker was mentioned, I'd go
with localized heat - some of the Loctite (e.g.) formulas are designed
to soften when heated in this way.