Victor,
I'm surprised no one has Replied to your question by now, but I'll try to help with what I know.
The Kohler opposed-twin engines have an aluminum crankcase with removable cast-iron cylinders that are held to the crankcase with six bolts each. The Briggs & Stratton opposed-twin engines have a one-piece block where the cast-iron-liner cylinders are permanently made as part of the crankcase. The heads of both engines bolt to the cylinders.
I personally prefer the Kohler engines, as I feel they are better engineered. Yes, they're more expensive, but that is the result of the better engineering. They're also more durable.
To use them for racing, the B&S governors are easier to remove to prevent internal disintegration when revved beyond the normal governed speed. Kohler opposed-twin governors, on the other hand, require a lot of engine disassembly to access the governor-gear/flyweight assembly for removal. But this is probably a moot point, as most racers will be disassembling the engine anyway.
As to upgrades, I'm not familiar with any commercially available ones. My philosophy has always been that if I need more horsepower, I get a bigger engine. Trying to circumvent factory engineering to gain more horsepower usually results in decreased durability. Of course, there are always the standard tweaks such as porting and polishing the intake and exhaust passages; maybe stiffer valve springs to allow higher RPMs; larger carburetors; tuned exhaust system; etc. And you can always groove the heads using Somender Singh's method which allows better combustion burn for more power. (This will work on any L-head engine.)
Hope this helps...
Respectfully,
Rodney Rom
Rom's Reworks
Butler, MO