Needs around 180hp plus 3% for each thousand feet he will normally cruise at. So say 10,000ft normal cruise altitude 180hp + 30% = 234hp which is like a Continental O-470. Or 15,000ft then he needs 260hp or a Continental O-520 or a Lycoming O-540. Or a smaller engine turbocharged to compensate for the 3% loss of power with each thousand feet of altitude but will still cruise at altitude with 75% of rated power or less. Or 2 or more smaller engines that will give equal power to above.