I don't really see 70 hp on a big block as a big ask especially if machine can do with a much lower amount. 50 % would probably be fine for cruising around and bigger power would be short periods. If cooled well it should not be a issue, add some quality internals, a bigger sump volume,a quality light weight motorbike oil cooler , a good flow job and balance, quality carbs- two not one. Add higher comp pistons, springs and retainers, coat the head internals and piston crown for heat, pick a cam to suit your rev range and torque curve. Make a quality header set and it should be able to make the 70 hp and not be a worry for a aircraft. And add a quality (not cheap) ignition system that makes lovely fat sparks. 70hp a litre max is not real big, in fact no more than a stock Moto Guzzi made. At 60 % its making 42hp at 70 % 49hp. So the 49hp with a quality build and parts should be fine as long as the heat is managed. That is more than a big block is max power approx but I expect it would actually spend time doing a lot less in a aircraft with Duncan. If 50hp was Ok, then thats say 35hp cruise, so a hot one will hardly be working its guts out. A well modded one should be smoother, use a bit less fuel for a given power, run cooler and be much more reliable. Would you scream around at max power? not likely. At reasonable loads and revs when replacing a normal but heavier version with a light , smooth and efficient version with more usable power- it should be a good thing. The only downside is labour and parts costs to do it right, but still that is not excessive. If it cant cruise at 40 hp all day, I would be very surprised.