I would think the Amal would pass more air for the bore size.With the slide wide open you only have the needle in the air flow.
With the Linkert wide open you have the venturi reducing the 1& 1/4 (1.25") down to 1& 1/16 or 1& 1/8 .
In the airflow you also have 2 spindles,the knife edge of 2 discs plus their retaining screws and the fuel tube sticking up in the middle.
If you added up the total cross sectional area of that lot your 1& 1/4 nominal bore is prob down to 1 inch or less.
Like you I've only done Brit iron before but now got into V twins.I would go with a Linkert (even if not the correct one to start with) you can mix & match parts just like the old Amals.As with the V twin engines I've found it a breath of fresh air and I'm fired up like I haven't been for a long time.
Get the Victory Library Linkert book and have a play.There's loads of spindles,discs and bushes out there NOS or repro from 45 Parts Depot.
Just steer clear of M88's and vented venturi (you'll box your self into a corner.)
Just as a taster ebay item #190063821695 about 12 hrs to go £52.38 +
£14.21 air mail.At those prices you prob wont pay any duty or vat,postie will bring it right to your door.
Sorry lads,don't know what came over me,but I feel better now.
NOTE to Bill Gates.Never mind XP & VISTA, give me a keyboard with fractions.