help 95 trx300ex just bought

I am having lot problems with bike I just bought.the guy said he rode every day then it started poping cracking so he put new ring 1 to 2 weeks later same thing so he decide tgo sell I bought got home new bat and valve adjust m/ent egual backfire but no start so tore down adjust timing then tried nothing u hold hand on carb it [email protected] ur hand in and leaves gas ring on hand so I tore down again but further and found piston to be in bad shape real bad groving on pist and piston wall how to fikx with out making the b ike much more p?owerful unless its cost about the same