race kart..
clutch!
the TC will be too large, too heavy and costs at the very least 20% of your power
(lost in belt slip by design)
for a fun kart a TC will have some benefits however..
you can drive slower easily, and you could climb some serious road inlcinations..
but frankly.. mounting a series 40 on a race kart chassis is all but trivial.
And NO the backplate crap is NOT an option!
a true jackshaft far enough behind the engne is the only way to go
Mounting a series 30 might work, but will be expensive since you'll be going through belts a lot faster than anticipated..
So yeah.. a cent clutch is your best bet.
oh a race intended one makes more sense if you're planning on modding the engine;
look what Noram has to offer for example.
'sid