IF you feel you want to sheet the open bay areas, I would suggest using 1/32" balsa (or 4 - 6 lb grade 1/16"). MAKE SURE IT IS SECURELY ATTACHED TO THE RIBS and use a .5 - .75 oz fiberglass cloth applied with MinWax PolyCrilic (SP ?). It is the water based polyurethane. Use just enough to stick it to the wood and let it dry (too much and you MAY warp the thin sheeting). Then give it a second thin coat. After it is dry, sand with 320 - 400 wet/dry doing it wet. Make sure you get the residue off, and when dry give it another coat of the poly, but this time mix micro baloons in with it. This will fill the weave of the fiberglass. Sand lightly with 320 - 400 wet/dry wet. Give it a final coat of the straight poly, a little heavier this time though. When dry, do the 320 - 400 bit again.

I would then spray a LIGHT coat of primer/filler (automotive type) and when dry sand it off to show any high/low spots. Repeat the primer/filler/sanding untill satisfied with the surface. BE CAREFUL YOU DO NOT SAND THROUGH THE POLYURETHANE AND FIBERGLASS. Clean the surface with rubbing alcohol and spray it with latex.

On a 40 - 60 size plane, this "covering" treatment will add about 6 - 10 oz to the weight (depending on amount of primer/filler used and number of coats of paint) of the plane, but will also provide a HARD, DENT RESISTENT SURFACE. Let the latex "cure" about 2 weeks (depending on humidity and temperature) and clear coat with waterbase polyurethane (fuel resistant to 10 - 15% nitro) OR LusterKote (or the UltraCote clear).

I am in the process of doing a SkyShark Hawker Tempest this way. It has mechanical retracts and will be powered by a TT 61. I anticipate my final weight at 6 3/4 - 7 1/4 lbs.

As far as the engine goes, that should be determined by your final weight. While there are exceptions, the GENERAL RULE is go 1/3 larger when going to a 4 stroke (i.e. plane calls for a 60 2 stroke, use a 90 4 stroke).

I hope this will help.