you missed a very
very important part with the braking calculations!
the tyre width and therefore tyre patch that is in contact with the road;
tyre/road friction if threshold for sliding friction is met or not (which unfortunately is a fuzzy line

)
since without static ( or in this case rather rolling friction) the stopping power will be diminished
by a fairly large amount.
Surely depends on exact tyre material (assuming slicks.. otherwise + tread pattern)
but a good estimation can be taken from the tyre's rockwell hardness
that's why different tyres on the same car have different stopping distances..
same is true for acceleration break traction and all power is heat
moving on to engine calculations.
kart speed or say top speed is mostly limited NOT by engine rpm but by air resistance since it is the fastest growing number in the resistance formula (being cubed)
so 10m/s end up as a factor of 1000, 11m/s yield a factor of 1331..
your 91 km/h (25.2778 m/s) end up as a factor of 16151.6844
so wind resistance coefficient times front facing area must not exceed 1.19 or else you motorpower is no longer sufficient to reach the estimated 91 km/h
that's actually quite good estimating a frontal area of less than 1m² you'll be fine even without fairing
You still want to be sure though.
tyres.. in general are smaller than their nominated diameter
(they flatten on the underside

)
so hop in a kart with the intended tyres mounted (properly inflated)
and measure the distance between the axle center and the floor.
that's your tyre radius for good calculations!
haven't gone through every calculation didn't even check consistency really..
I noticed two things however..
you pull numbers out of thin air every once in a while
like the 746 [watts per hp] and worst the 11hp @8000 rpm
which is bad in papers like these..
be precise! it's rounded 745.7 Watts per bhp (.69987)
and where you got the 11horses from @ 8k rpm I have no idea (not having any engine documentation handy and being too lazy to search for some)
but it's rather safe to assume if the engine peaks at 11 bhp @ 9500rpm, the output power @8000 rpm will be slightly less than that.
But yeah.. nice effort put into that indeed.
add proper documentation and referncing and it'd almost pass as
scientific
'sid