Ken is right.
Your best bet is to get a pre 1953 .22 Kit Gun 2" & 4" barrels, or the .22 Target Model with 6" barrel. You could have it satin nickel plated, hard chromed or refinished with Metalife in lieu of stainless steel.
Or just convert your Model 63 to a flat main spring like you M617.
you're not the first to have this wish. Is it just swapping parts? No, a combination of parts swapping and just a little hand craftsmanship.
Here's what to do:
A round butt J frame is much easier to convert to a leaf spring.
Photo 'A': Coil spring:
Photo 'B': Leaf spring:
STEP ONE: Secure a hammer w/toggle leaf spring mount, shown below, (or drill the hammer for the toggle and pin), leaf spring, and spring tension screw from a post WW II "Transitional" I frame (still uses the leaf spring). Rimfire .22/32 Kit Gun hammers must be converted to retain the wide spur target hammer; just requires drilling the hammer for the toggle and pin, and reshaping the bottom a bit. Can also be done on the center fire hammers.
STEP TWO: Remove coil spring and swap hammers.
STEP THREE: Using a jig saw or coping saw with metal cutting blades, cut out the coil spring mount from grip frame shown in 'A' above.
STEP FOUR: Drill the hole for the bottom end of the leaf spring at the location shown in 'B' above. Cut the slot connecting the hole to the open space.
STEP FIVE: Drill, tap, and countersink the hole for the spring tension screw. Install spring and screw and there you have it!
Parts are available on ebay and gunbroker; search for a pre 1953 I frame parts kit.
NOTE: Parts from a pre war I frame with the trigger rebound slide (post ~ 1915) are more numerous, easier to find, and useable but the hammer will need a 'flat' filed in the face below the firing pin to work with a the post war frame which has the sliding bar hammer block safety.
