RE: 4 stroke selection OS, Saito or ???
You will probably do best with the brand most popular at your field. You will then be able find help without someone running down your engine. OS and Saito are both good if a little more expensive than Magnum or Thunder Tiger. If money is an issue I would go with the Thunder Tiger. My feelings on the YS is leave it for the "expert" pilots that feel they have to have what they consider the best. I think they are more finicky, require more in the way of servicing (i.e. bearing replacement and pump work), and are heavy. On the other hand the pumps do offer a consistent and even running engine and this is one of the things the competition pilots really need. Remember, when competeing, money is no object and the best pilots will spend oodles of it for marginal performance gains. Refer back to the first sentence, it is the best advice I can give.
Merle