Unfortunately flight training tends to teach engine failures as binary, engine is either working perfectly or it has suddenly and totally failed.
However sudden complete failures with no warning is the least likely real world scenario.
A partial loss of power is a much more likely occurrence. I specifically address the partial engine failure in my training starting with a discussion about knowing the minimum power required to maintain level flight. Obviously a deterioration of
power below this value means you are no performing a power assisted forced approach.
Stable power above this value now gives you some real options and so you need to decide on a where you are going ASAP. This would normally be the nearest airport with landable terrain between you and it. Once the situation has stabilized you would then do a cause check to see if you can identify the problem and restore more/full power.