You won't die doing either of those manoeuvres in a PA38.
The loop will shag the engine because its not built for it but the aircraft won't drop out of the sky or the tail fall off. Students recovering from spiral dives when the speed gets up, pull way more G than a loop. Its a function of the oil system on the engine amongst other things
Barrel roll in the right hands (not mine I may add) will not do anything at all to the aircraft. Allegedly Concorde did one may years ago.
Its a matter of the certification. As I said before I am sure these manoeuvres have been done before but I certainly wouldn't recommend it and I will never be in a PA38 when it does them.