Agreed. Weber 32/36 for budget, twin Weber DCOE40 for ultimate sound/performance. Choose 40s rather than 45s for best power & flexibility with an A12.

Another good choice -- the best for fuel economy -- is the Hitachi DCG-306 (manual choke) or DCH-306 (auto-choke). Brand new aftermarket versions are available from Bprojects.

Your A12 with 75mm pistons is same displacement as the factory A12A engine (1237 cc). A cam advertised as a '70' does not usually mean mild, but are high-RPM (7000+ RPM) with a lope to the idle (rough idle). It is a couple steps above GX camshaft (6400 RPM).
example:
Nismo 13001-H5710 70° camshaft 280° duration
* recommended for rallies
* limit RPM 7600, normal RPM 7200
* lift 9.4 mm

For 7000 RPM operation the stock carburetor will be a chokepoint. Instead get a 32/36 or a twin-carb setup.