'XXX' I think you will find that yr comments re max power limits only apply for certain engines. The basic Lyc O320 for Eg can run flat out all day(in fact I think that's a req for running in a new donk, but don't quote me) providing it doesn't exceed certain parameters such as prop RPM Cyl head temps etc. I don't recall seeing a max power time limit in any of the basic machines I used to drive a 100 yrs ago:-)
Our resident engine guru 'Jaba' would no doubt be able to provide volumes of info regarding such engines
Wmk2