Rebuild the OEM carb; your aftermarket carb probably doesn’t fit the starter valve and your choke (enrichment) circuit is open. Your plug is probably fouled too
https://www.hondaatvforums.net/forums/honda-fourtrax/119242-trx300-carb-rebuild.html
https://www.hondaatvforums.net/forums/honda-fourtrax/119242-trx300-carb-rebuild.html