Ok, first of all I would recommend that you do a search here and on any other RC forum you can find for the SeaMaster 120. With few exceptions, it was considered a very poor flyer. The kit was discontinued after the first production run. If you decide to go ahead with the build, there are some fixes out there that you should definitely use.

If you still want to proceed, you will probably need 70 ounce servos (HS635) on the ailerons. One for each aileron will be sufficient, you do not want to get into buying matchboxes to balance two servos on each aileron. Using a single servo each for the elevator and the rudder, you should consider something in the 100 oz range (HS645).

A high landing speed was not one of the SM 120s problems, but weight was, so I would say that flaps would probably be a bad idea. Still, if you want to install them, use HS 475s.

And finally, treat that 180 four stroke with the upmost respect. A 91 can cut you badly. A 180 can amputate.

Jim