RE: Avistar four stroke
A Magnum or O.S 52 four stroke would be good choices. If you have a very fat wallet, the new O.S. 56 four stroke would also be a good choice, other than the outragious price. You can get two Magnum 52's for the price of one of those. If you really want performance, a 70 would work, but it caries the penitaly of extra weight.
Don