I had a 18 hp Opposed that ran great.. and I revved mine for a whole season.. BUT.... remember how big that flywheel is.. and if it came apart it would do some serious damage.
If you want to run it ungoverned I can give you some tricks from the motor I used to run 3rd in WOO points in 2008. I would see if midwest's V-twin billet flywheel could be modded to run on the opposed Kohler. I would hate to see you hurt.
Great torque engine and I ran mine to 6000rpm with a 7.90 gear for the entire season. Not enough to pass the Ryker vannies but enough to hold on for a heat win and a bunch of 2nds and thirds in the feature..
The 18 Magnum weighs 140 lbs dry weight..