I have a microwave oven with a busted timer. It works fine in all other ways, it's just that the rotary time control doesn't un-rotate, so the m/wave runs continuously. It's an extremely cheap no-name one from Sainsers (*). (Who replaced it, even though it was outside of warranty, so we have the broken one spare, now).
Is it even worth trying to repair? IIRC, it only cost GBP24. Can you buy the timers? A Google search is unsatisfactory. It's presently on the pile of stuff to go to Emmaus
(* Purchased for my 90 y/o MIL. We weren't sure she was going to like it, so got her a super-cheap one.)