I have recently converted a Norton Dominator 7 " centres SLS to a Manx 4LS (for racing) and offer the following.
Firstly and probably most importantly a replica Norton 4LS front brake is
seriously expensive. Together with the other changes you need to make you will not have any change from US$4000 plus.
If you want to look at cheaper options you could look at alternatives on this site.
http://www.discovolantemoto.co.uk/brakes/cat_3.html
If you go the Manx 4LS route Andy Molnar is indeed one of the main suppliers but I actually bought mine from a friend in Australia.
http://www.eurospares.com/greg.html
I consider his option to be better in the mechanical details but thats only my opinion and Molnar's are used everywhere. My Ozzie brake has been used by Chris Swallow on NZ street circuits and he gave it a big thumbs up. (son of Bill Swallow 9 times Classic Manx winner. Chris has also had good placings in the Manx)
Further considerations.
You will need to buy Manx sliders (from Molnar) because the brake anchor system is different than a Commando. You will then not have the holes for Commando mudguard struts. The Manx sliders will fit on to Commando forks.
The 7 3/8 versus 7 " centres is not a big deal but you will have to make up spacers and longer brake anchor fixtures.
The 4LS brake has big airscoops which will not suit everyday use in the rain.
The brake is magnesium but you might be able to get an aluminium replica made on special order.
The brake does not look like a Commando brake.
These are racing brakes and vendors will expect you to be able to set them up right. Arcing the shoes. Remachining the liner after the hub is spoked into the rim. (or buy a whole wheel already set up)
They are the top end of the market and they are neither cheap nor fit and forget
Regards
John