A well built engine of that era should last a few thousand miles.
Even the old Ford cosworth used to run 540BHP in RS500 Race Spec and over 630 in qualifying spec and that was a crappy design!
The old M12 1.5L BMW engine was good for 1500BHP in the right trim
A small capacity High output engine is always going to be more stressed compared to a similar design large capacity engine.
But modern tolerances, stronger build means that higher power can be achived with better reliability..
But 1000BHP from a 4Pot is not going to give you 10's of thousands of trouble free miles though. But it's not what it's for!
TT