JOSH - A harmonic balancer puller from a car engine works good for pulling Kohler flywheels. Don't know what it is about K321 flywheels but they do come off hard! I remember taking the one off My K321, I let the impact hammer & hammer on the puller, tapped the flywheel, tapped the puller, used a BIG breaker bar on the puller. Finally let it set for a while when I made an "Adult Beverage Run", it must have moved when I was gone, little 12 inch crescent wrench tightened the puller screw a bit and it popped right off. Don't drop the flywheel when it does come loose! It's heavy too.
KRAIG's correct about making your own puller, in fact I'd use something thicker than 1/2" steel. I have a piece of 1/2" x 2" steel in the shop all machined with 1/2" fine threads on the puller screw but it's been bent once or twice already.
A local autoparts store like O'Rilley's, NAPA, Autozone, etc should have a loaner harmonic balancer you could "Rent for free", costs some cash up front but it's 100% refundable.
Ohhhh and use good bolts, grd #5 at least to thread into the flywheel, think they're 3/8" coarse thread. The harmonic balancer puller probably has an assortment of usable bolts with it.
You knotching the frame on your 124 to fit the K321 in or making a "Small flywheel K321"?
I really like my small SFW K321, the large & small FW weigh the same, but it's amazing how much more responsive the small FW makes an engine. The K301 I "borrowed" My small FW from turned into a real dog with the Big FW installed. Still pulled good, just wouldn't accelerate.