That's pretty good. I guess it is a 42'' fuse without a fin. 55'' fuses with fins weigh 16-20 oz. An 8 oz fuse would be ideal for a sub 4 lb model.
The fuse is 38" long as measured.
Sure, it's meant to be fast but also smooth and graceful. 45's and 50's carry 5-7 lb models so I can't really see a sub 4 lb model with a 42'' fuse and 49'' wing having a 45 shoehorned in there. IMO, the wing just doesn't have the squares to carry 5 lbs or more in a classic pattern style. What are the root and tip chords of the wing? I have a glass/foam Blue Angel kit with practically identical specs. The model is designed to fly with a 25. I plan to put a piped 25-32 in it with retracts.
The root cord is 10 1/2", tip cord is 6", length is 24". By my calculations total wing area is about 400sq. in. less ailerons and tips. Stab root is 5 1/4", stab tip is 2 3/4", area is approx. 77sq. in.
Note that if the model is built to sub 4 lbs, you can use light 5 oz trike etracts on it. Either way, I'd build a very light core wing, stab and fin and see how she turns out weight wise. You can always build a wood fuse if you want to preserve this one. I'm fairly confident that you could wind up with a finished 2 lb airframe - painted fuse/fin and covered flying surfaces.
OK, a light just went off. I kept asking myself why you keep harping on a sub 4lb airplane so I did some math. Correct me as necessary.
400sq. in = 2.78sq. ft
A 4lb airplane has wing loading of 23oz/sq. ft.
At 4 1/2lbs, 25.9oz/sq. ft.
At 5 1/4lbs, 30.2oz/sq. ft.
looking at a sport plane like an Ultra Sport 40 shows a wing loading of 24oz/sq. ft. Given this, I would definitely want to be at 4 1/2lbs or less.
So what would be a good wing load range for a good flying model? There must be a balance. I think if it's too light it wouldn't handle wind well.
I have a Magnum Pro 45 which is 2.5oz lighter than the YS45FR. Maybe this would be the way to go.
How much do they weigh?
A YS45 is 14.1 oz and a Magnum Pro 45 is 11.6 oz (329g) w/o muffler.
An OS 32SX is 9.5 oz but the Magnum 45 would have about 50% more power.
Like I said before, the Magnum 45 is an OS 45FSR copy. The 40/45FSR engines are the same size and they are slightly smaller than a 46FX and about 1 1/2 oz less weight.