Have a look at the wiring diagrams in the manual. For the generator, converted to alternator, cars (both RHD & LHD) the diagrams show no ammeter (or voltmeter) and a splice connection is shown where the meter was shown in the previous years diagram. In the diagram that covers the cars from there up to the +2S 130 cars it shows the battery condition indicator (voltmeter) wired between the B+ connection on the instrument voltage regulator and ground.
I would not assume that the ammeter circuit is the only change in the wiring and order a replacement loom for an alternator car without checking everything else out. On the +2, it's hard enough to find the right replacement harness for your car without trying to find one that might be the one for your modified car. Read a few threads on wiring harness replacement and you'll see what I mean. Also look at the five different choices offered for harness sets by
http://www.britishwiring.com/default.asp or the other suppliers.
My March built '70 +2S, although a US car, started out a a generator system and like yours was converted. I think mine was converted by the other owner before me. The ammeter was still wired in and, if the battery was too low, spent awhile pegged at the 30A end until the alternator caught up. I'm in the middle of a complete rewire and replaced the ammeter with a Smiths Classic voltmeter which will look right at home with the other instruments. It's wired in with 18AWG wire and not 10AWG unfused.