Just a couple of thoughts.

Will ALL a14's fit / no mods etc?...
No, you will need to add, change, or use "some" bits from youe old engoine, like thermostat housing, or water fitting at the back of the head, or perhaps you will need to use the braskets from your A12 block for the engine mounting. Generally speaking, if you can pull the A12 out, you should be able to get the A14 in.

Extractors?
You should be able to use them OK

Webbers, wasting fuel?
If you fit them, & pay to have them CORRECTLY tuned for your engine, you will find that any fuel that is wasted, won't be the carby's fault. If they are NOT correctly tuned, you will wish that they were.