As has been said before, Molnar Manx frames are to factory drawings. As are his yokes. I’m pretty sure the same it true for Ken’s stuff.
Molnar strongly recommends 18” wheels.
So 18” wheels work very well with standard Manx frames Al.
I do wish you’d recognise this.
Molnar Manx Nortons probably have 60 degree rake on the steering head as do most after-market featherbed frames these days. They probably also have less yoke offset compared with a Commando. An original Manx has 54.5 degree rake and about 2.25 inch of yoke offset and comes with 19 inch wheels. The bigger the wheels, the more trail you have. The shorter the yole offset, the more trail you have. On an original Manx, the 54,5 degree rake decreases the trail. But it all balances out to become a very nice handling bike. A similar result can be achieved with 60 degree rake, 2.25 inch yoke offset, and 18 inch wheels - which is probably what a Molnar Manx has. When you fit 18 inch wheels in place of 19 inch wheels on a bike without changing the rake or the offset, you decrease the trail. Reduced trail causes understeer, both going into corners and coming out of corners. It also increases the lean of the bike in corners.
The increased lean means you need better rubber, but better rubber is the reason you fir 18 inch wheels in place of 19. If you had better rubber with 19 inch wheels and not 18 inch, you would be much faster in corners, because the bike stays more upright and tends to oversteer, so yo can gas it much harder in the corner.
Ken McIntosh uses Molnar motors in featherbed frames with 19 inch wheels. I would almost be certain his frame rake would be 54,5 degrees. And his fork yokes would be the old Manx offset of 2.25 inch. Commando fork yokes have more offset than Manx yokes, Molnar or original. So less trail and when you fit 18 inch wheels you reduce the trail even further.
All that happens is the bike becomes stiffer, leans more and does not turn so easily. Which on a road bike might be good - OR NOT.