my friend would replace it with a modded predator or something along those lines.
A big problem you'd encounter with that plan is that frames angled rear cross-member...
Most all 4-cycle kart engines (like the Predator) have their clutch on the left-side. Most all 2-cycle kart engines like that KT100 on the right side.
That means you'd need to instal the axle sprocket in-board of the axle bearings for a Predator, and that frames angled cross bar isn't leaving enough clearance for the 8' to 10' diameter axle sprocket & chain you'd need in that location.
You'd need to instal a Jackshaft or mod the frame to instal a Predator.
That frame has a shift lever, it probably used to have a Honda CR-80 or 125 engine on it. The drive sprockets on those engines are on the left side like a predator, but they use a much smaller (3"-4") diameter axle sprocket that wouldn't cause any interference with that frames angled cross-bar.
I wouldn't pay more than $300 for that kart. Even assuming the KT100 engine & clutch are in good condition: by the time you buy a kart-starter, exhaust, motor mount, brake caliper/master cylender/brake lines, axle bearings, seat, pedals, fuel tank, nerf bars and rear bumper, you'd have way more invested than an older 1990's-era racekart like that is worth.
If you shop around a bit more, you'll probably find a complete-running racekart like that with a Predator priced between $300-$500
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