Should I build this glider?
Depends...
If you already have the building and flying of a couple or more models under your belt, you'll probably bored with this one, it was a beginner's model.
Is it worth anything if left unbuilt?
No idea. You could try putting it up for sale on Ebay.
Can modifications be made to have elevator or full stab control?
Probably. With enough work and material anything can be modified
. The radio gear and servos of that era were bigger, heavier, and less powerful than what we are used to today. So probably you would have no problems fitting 2 Hitec HS-81's or something similiar in the fuselage. Elevator : Just make a strip of balsa the length of the horizontal stabilizer, width : approx 25% of the stabilizer chord, hinge it, and presto ! You have an elevator !
Michael