Well the plans show a very rugged construction...a bit too rugged in some places to my preferences. The wings are covered completely with 1/64" plywood for instance.

The advertised weight is 21 pounds. If one takes care when building and doesn't cover the entire plane with glass, this weight is probably achievable.

On a 21 pound plane you don't need two ST 2500's. Also fuel consumption will be very high, so you'll need a big tank...
A couple of 1.20 4-strokes would do the job more then adequately. If only 21 pounds, the plane would go almost vertical with 2 1.20's...
I'm thinking of Laser or Saito 1.20's.
How about a couple of RCV 1.20's? With these you could turn these nice scale props...there's room for 20" props on the plane (according to the plans).

About the cowls: if there are more people building the plane it would sure make sense if one of us made a nice set of moulds. I haven't got much experience making female moulds though. For the one piece job I usually make a foam plug, cover it with glass and remove the foam afterwards...