You have the correct 250 motor.
Polaris used 340 bottom ends for both the 250 and 340 motors.
If you still have the tag on the front, PTO side of the crankcase go by that ( the earlier tags were printed and could get scrubbed back to just aluminum). As well as the cylinder cast number.
For your brakes, remove the hand lever, pull out the piston and clean everything with a Q-tip. Make sure both the big and very small holes are clear. Get the jelly out, refill and make sure to bleed it well. You'll probably be fine then.