It's not at all uncommon to see folks replacing the single ZS with a complete UK-spec. dual HS4 SU setup. These setups tend to be fairly available from some UK Triumph suppliers, eBay (of course) and folks like Ted Schumacher in Ohio. Just bolting that on should see a reasonable improvment, although other improvments (cam, distributor tweaking, etc.) certainly would help more.
Similarly, even the Mk3's dual HS2 setup will bolt up and probably yield a similar improvement. But those setups are getting to be almost as scarce in the US as the above setup (which was never available on US cars)!