Ok, then he might look for something really small, light, reliable and powerfull. May I dare to suggest a couple of Norvel .061`s, or even the Norvel .074?? They are really compact and much lighter than a .10 or .15 but produce about the same power. These engines are a little difficult to obtain, but show up on the "big auction" site from time to time.
I fly a Norvel Big Mig .061 and it has the power and throttle response to easily hover my 400 gram 3-D plane down on the deck, totally dependable

Tomorrow I will test my AP .061 in the same plane. I got this cheap chinese copy of the Norvel to find out if it is any good, and can replace my Norvel after it is gone. It runs great on the bench and I`m hoping it will approach the Norvel in terms of power and transition