I think I might be on the way.
From a piece of ½" steel plate from the centre plate of a Purbrook Heron (don't ask), I've managed to make a 'sort-of' faceplate.
Took a lot of hacking and angle grinding.
But with four holes in it each big enough to take the 'tip' of the four jaw jaws, if you follow me, then drill a hole in the middle as big as the largest drill I have, and bore out to about 1 ¾" so it can be mounted in the three jaw and faced off etc.
So now I've got a 7" faceplate that mounts in the four jaw chuck, and can be jiggled to get the flywheel running true to the inside edge of the rim.
All it needs now is drill and tap some holes in it for clamping bolts for the flywheel.
And take things very carefully.