I use light weight spackle for a filler, the tub it comes in weighs more then the product. I let it set up over night then sand it smooth then just give it a bit of thin CA and it becomes part of the wood. Once it has been CAed it doesn't sand though.
I used the OS .46la engines in all my fun fly planes, all of the planes were the Up-Roar then I used them in several different planes over the years. They are great little engines and last forever if you use a good fuel with Castor in the mix.
However the FX and AX are a better product and produce more power and if you live long enough to wear one out they are an easy rebuild but the LAs are a bushing type of engine so when they wear out you pretty much just toss them.
I have known people to rebush them but for the price they aren't worth the trouble.
In the small two strokes any of the OS engines are a good choice. The OS are pretty much trouble free and backed by a good warranty. There are other brands though but for a first engine I would stick with the OS.