Mr. Charley Davidson:
Only thing I see bad about the idea is the HP and the work. But it could be done. My 820 is powered by a 3/4hp motor and while it is not under powered it certainly isn't overpowered either. Been thinking about robbing a 3/4hp vfd and motor off my wood lathe and using it on the Logan. Had to have the wood lathe and now that I have it I very seldom use it while the Logan hardly cools down.
If you take the motor off the gearhead and mount it on a plate that bolts to the "C" face motor and has a 90 degree base plate to bolt to the base of the lathe. You could make the plate or just buy one as they are pretty common. Ask them at the bearing shop and they will find one.
Resize the drive belt and a little control work and you are in business. Fun project. I have done a few of these for guys who have Chinese hobby lathes and they all brag about the difference it makes. Guess I'm gonna have to do it for myself. Just as soon as I get my Cincinnati #2 running. It came in straight 480v so we had to change that. Adding a VFD to get my 230v 3ph for the wheel motor and the table motor is 90vdc so a little transformer action there and we are in business. Just more fun. From what little I have seen so far that Cincinnati is one helluva stout built machine. The tolerances on it are so tight it is like everything is stuck just from dried oil after sitting for the last five or six years. Gonna be fun.
The price you have on the VFD and gearhead is very reasonable. I think what is shutting you down is the gear head portion. Either people don't understand it's application or it's over their heads.