K125 with box muffler makes less power than a KT100 4 hole so about 10hp. K125 with pipe makes a little more power than a KT100 4 hole so about 13hp.
That engine has small transfer passages, low compression ratio and needs a good helicoil for the spark plug and maybe a slightly bigger carb. Fix that and it will go a little faster. You're not going to get too far because it's got a massively heavy piston and it craps the rod out if you push it. Keep it stock with box muffler on pump gas and it will last forever.
Sundog