Re: now a gear problem
You should be able to adjust the shift cable under the cowling, you don't sa ywhat motor you have but mine is on the starboard(drivers side) there are 2 cables, shift and throttle with knurled knob on each one.
Find the shift cable and verify which way it moves when shifting into reverse and turn the knob that way.
You can correct a small amount like this altough if it shifted right before you cracked the case there may be a bigger problem.
What is happening is the gears are always turning if the engine is running, there is a small pinion gear on top with forward and reverse facing that like an upisde down U and a clutch dog in between, when you shift you slide the clutch dog forward or back to engage a gear and it isn't sliding far enough to engage and the ears are jumping out of the gear.
If you can't adjust it out there may be a bigger issue.