That depends what 300 your are refering to. If it is post 07 it would be called a trx 300x pre 07 trx 300ex. The two non sport quad 300s and 300SW 4X4 and 300P Fourtrax. He is right thogh if it is from the utility bike It's not gona work. I would just ask if its got a shaft or sprocket on the side. If it is from the sport quad than you should be in good shape no mater what year it is. Only problem a 300ex's wireing is not the same as a 250x.
Truly, sir, in respect of a fine workman, I am but, as you would say, a cobbler.