A lot of people use a rear end from an ATC Big Red 250 or a 350 Fourtrax/ 350 Foreman (86-89) in a 300. Those rear ends have fully tubed rear ends with bearing support out on the ends of the tubes, so the rear axle is better supported and much more resistant to water getting in the rear end and eating up bearings and eventually gears.
All that is required to swap them in is an adapter plate that welds onto the 250ES/ 350D axle tube to match up with the 300 swingarm mounting flange. The diffs themselves are the same gear ratio and bolt right up to the swingarm.
you forgot to mention that the rear brake cables wont be a simple hook up..lol.
'89 trx350D foreman 4x4...'85 vf700c magna, '03 trx450fm foreman 4x4 , 2015 Yamaha waverunner fx cruiser ho, '08 vt750c2 shadow spirit