SU is potentially useful on a 45, but has the same problem as any other CV carb - which needle do you use? There are over 600.
Mr. Smith, of course, goes on at great length about the carbs but says something rather foolish "I've never run into a case where the SM (needle) wasn't best", strongly suggesting that his experience was limited to the stock TR4 carburetor. This is hardly a "properly tuned and calibrated carburetor".
He also does not discuss spring tension.
He also says that there are only 3 sizes, completely ignoring the H1 and H2 which are more suitable for the 45. The numbers:
If "-" number is 1 digit, take "-" number, multiply by 1/8", add 1". -6 = 1-6/8 = 1-3/4", etc.
If "-" number is 2 digit, it's the size in mm: 38mm etc.
Known sizes: 1-1/8", 1-1/4", 1-1/2", 1-3/4", 1-7/8", 2"; 38mm.
There's even a special HIF6 for blow-through turbo use (Leyland Turbo Metro).
For a 750-800cc twin (based on fairly low-perf auto engines) A.G. Bell suggests for the H4 etc.:
CZ or QA (slightly richer on top) both with red vacuum piston spring