Well, great project !

First, you have to imagine the size of your future plane. You know the thrust of your engines, so, lets take a thrust/weight ratio of 0.5, that is a good value for a liner jet.

So you have twice 19 pounds of thrust, that should make a plane of 76 pounds. Now you have to found the scale that produce a plane of 76 pound of final weight approximatively.

For exemple, have a look at Aviation Design Falcon 7X here : [link]http://adjets.free.fr/adjetsv6GB/falcon/falcon.htm[/link] This plane is 69 Lbs weight for a single Jetcat P160SX turbine. That's near your though...

EDIT : The AD Falcon 7X should be a good subject for a twin turbine conversion with twin "90 size"...