Small Engines (Lawn Mowers, etc.): B&S V-Twin, side draft, v twin

Question
My question is a little out of the ordinary:  I just bought a used Cushman industrial vehicle with a 14 hp B&S Vanguard V-Twin and converted it to off-road with taller aggressive tires, etc.  I like the new top-end speed from the tall tires, but need more torque.  I noticed this same engine design comes in 16, 18, 20, and 23 hp s.  Do you know the basic difference between the 14 and 16 or 18 hp s?  Is it just bore and stroke (I can't get specs on the internet) or possibly carb and intake?  I found a tech drawing that shows a two barrel side-draft carb and intake, while mine has a one barrel down-draft. Any suggestions to get more torque from the engine without changing the gear ratio?

Answer
the major difference between them is bore the only way to get more low end torque would be to add more rotating mass which will slow the top end down. you can build it as you would a race engine. but you are talking custom made parts, and quite a few dollars. the cheapest if you have a friendly machine shop, is to convert a used turbo charger to a belt driven blower. which i have seen done but it will take its toll on the engine.if you want to pursue building it let me know and i will tell you how.