Been there, done that.
First grab a strip of abrasive cloth and give the crank snout a quick polish, then look for high spots-they will be shiny after polishing with abrasive cloth...carefully file any high spots down with fine tooth file.
Examine center bore of balancer especially inner edge that slips on crank...Look for small dents that might reduce inner diameter a little. File those away with fat rat tail file...Examine keyway in balancer for same and polish it square and smooth with small flat 4 sided bastard file with fine teeth
Crank key is most likely captive woodruff style that fits in half moon slot, right? If so, examine key slot in crank for dirt/burrs that might preven key from seating all the way down.
Once all the above is done, seat key in slot, check that it is parallel with crank, check height of key to make certain it will enter keyway on balancer, then get yourself a fine point center punch and small hammer.
Yep, you gonna gently stake the key in the crank snout at each side of rear of key.
Once assured all the above and key ain't too high for keyway in balancer, tap key lightly but fully down into slot in crank, making certain it's level front to rear....Now take pointy center punch and place point near rear of key where it meets crank...strike punch enuff to displace a little crank and key metal...dimple them into each other...do both sides.....now hit the punched joints with a light filing and try to slip balancer on
I balancer still stubborn, then heat in kitchen oven to 180 degrees, wrap crank in ice packs and in about 45 minutes try it again.
If balancer does not has rubber isolate core, heat it to 200...you can also spray crank with C02 extingisher just before slipping hot balancer on