Go to this link and tell us which carb that engine has?
Is it the dual throat 125 or the single throat 125?
Are you saying your Briggs twin NOW idles OK but will not rev up when advancing the throttle?
Really need to know which carb that engine is using??????????
Next thing about your Kohler with the bad crankshaft keyway.
When the tapered shaft is clean and the taper inside the flywheel are both free of oil and TORQUED properly the aluminum key is only used for placement of the flywheel so as the engine is TIMED CORRECTLY.
I've used engines without a ALUMINUM flywheel key by assembling and torqueing the flywheel properly. I've seen where some people (space cadets) even lubed the tapered shaft so as the flywheel could be removed easier next time. (bad idea)
The aluminum key is not to prevent the flywheel from spinning on the crankshaft. You were not cleaning/installing the Kohler flywheel properly is why it's now in bad shape.