I've soldered main jets before, it works fine.
May be some other options-
#26,28 or 30 kohler carb. Or a KT 17 carb. All are fully adjustable, but I don't know if they
would work for you or not, maybe someone here does.
If that carb has a removable main jet you may be able to use a high altitude
jet, should be a little leaner and you could drill it out some if you had to.