Why do you *need* a six speed?
imho, the 914 motors work great with the 5speed they already have (and heck, in 1970, not many cars had a five speed to begin with!) Between the 5spd, disk brakes and fuel injection, the 914 was extremely modern in its day.. its just that its day was 35+ years ago
I will say, I really LOVED my 2056cc 4cylinder 914. Mine was a 1970 and with a full tank of gas was 1990lbs. The 2056 put out approx 120hp. Easily kept up with my dads original 2.0l SIX. Great torque, good fuel mileage (mine had dual webbers).. reliable and in keeping with the spirit/lightness of the original car. No.. it wasn't a street racer, but it did have tremendous balance and at the track, many assumed it wasn't a 4 since it could keep up with 911s.
I say, take the spare 2.0 and make a nice 2056 out of it.. maybe even something bigger (2270?). They really waken the car up.. but they don't make you buy new transmissions, axles, engine mounts, oil tanks, radiators etc. They are true "plug and play".