Which Carburettor should / can I fit to my A7?

Good question, it all depends on what you want to achieve, how far from originality you want to go and be able to return to standard. The key components that need to be considered are;
•     The inlet manifold
•     The exhaust manifold
•     The exhaust pipe
•     The fuel arrangement pump, pipe and tank
•     Throttle and choke linkages may need to be adjusted for all options
 
Options:
Pre-1932 updraft Zenith Carb: if you wish to remain with the inlet manifold, the easy option is to fit a Zenith 24T2 carb, see photo. It is also possible to fit a right angle bend at the base of the manifold to take an SU side draft (below left), or modify the aluminium inlet manifold if you have a spare to take an SU side draft (below right).
The next option for a pre-1932 car is to replace the exhaust manifold to the post 1932 cars. But in doing this you will need to modify the exhaust pipe as the connection to the manifold is in a slightly different position.
Once you have upgraded to the later exhaust manifold you could then run with a Zenith 26VA, as on the later cars, although it may be easier to find an SU carb rather than the Zenith 26VA.
On the right is a Chummy with the later manifold and 1” (OM) SU. Note the carb float chamber lid has a tickler to bleed the fuel, same as the updraft Zenith Carb, and run the fuel without a pump from a forward petrol tank.
On the right is a 1930 Two Seater with the later manifold and 1¹/₈” (HV2) SU. The mounting holes in the SU need to be adjusted to connect to the manifold as the centres of the studs and holes are not the same.
Once you are looking at a 1¹/₈ “ SU it will depend on the orientation of the fixings on the SU horizontal or vertical. Horizontal can be adjusted to fit, vertical you will need an adaptor (see other article). If you are looking at fitting 11/4” you will need to look for an SU with vertical mounting and an adaptor (see other article) but a 11/4” is probably too big for a standard engine but if you have a non-standard engine then to get the benefit you probably need a non-standard exhaust and non-standard inlet manifold, at which stage you have moved away from a standard arrangement - there are lots of options. Douglas Alderson DA7C

SU Carburettor Development and Types
We are fortunate to have Burlen who have taken over the production of SU Carburettors, located just north of Salisbury. Not only do they have a useful website www.burlen.co.uk which contains a number of technical articles, they also produce a 179 page catalogue for free. Thanks to them for the information contained in this article and for the permission to reproduce the images, this article is a brief summary of the SU Carburettor Development in relation to one you might fit to an Austin Seven. In this article I have looked to focus on how to easily identify SU Carburettors and assess their age.
The SU Carburettor consist of the following key components.
•     The body
•     Piston and suction chamber
•     Throttle linkages and links and choke brackets and leavers
•     Jet assembly
•     Float chambers and lids.
 
With variations on the above components SU produced a number of different carburettors for different makes and models of cars (the Burlen catalogue lists 40 pages of cars with different SU). This variation of SU Carburettors can make it a challenge to find a suitable SU, but once you have the correct body the SU can be adapted to have the correct linkages and float chamber etc.
The body developed over time and hence some will look more period than others on an Austin Seven. Also it should be noted that some of the bodies come in different sizes, so it is always best to check the size of the body before you buy. The carburettor is measured on the engine side of the carburettor bore not the air filter side.

I have listed out all the sizes available so you can make sure you get the right size which is probably 1” 11/8” or 11/4”. Douglas Alderson DA7C