the appeal of the vincent is the motor, the egli frame or the stock is nice but isn't worth the big bucks. All in all the chassis is probably the cheapest part of the project.
HD motors won't work because they are long stroke big bore engines. The Vincent is more of a short stroke motor...and besides it is ancient technology. It isn't the preformance people are after when they use these mills, it is the asthetic beauty and the sound which can't be reproduced using another motor as a base. for 1952 a 50+ hp motorcycle was a big deal...but now there are ninja 250s that make more than that.
I remember it cost approx $10,000 a set of cases to build billet H2 motors, for vincent owners this seems like more logical solution than for any other japanese bike, espically when you can make upgrades like modern 6 speed cartirdge gearboxs and not have it affect the asthetic.
To be honest, while the egli vincent is nice, for half the money I would rather have the ducati 750 and 900 SS replicas that use the Monster DS aircooled motors. Classic looks, raw power, reliability and new technology. can't beat it.