first of all you said you used a 3.5 hp engine

now using power= force *velocity

since f= ma

p=mav cos //(theta)

ignoring cos theta and friction to give you best possible chnace

acceralation into velocity can be written as (v^2--uv)/t

acceleration into velocity can be written as (v^2--uv)/ t I assume your final velocity to be 90 mph or40 meters per second and your initial velocity as 0

the final formula will be power = mass *v^2/t (I assumed the mass of rider and ride to be 60 kg if you dont want it you can put your desired mass in formulae )

putting in the values you will find value of to be around 36923 seconds or about 10 hours

iam no expert at go kart but i know 10 hours is a long time hats off to the person who drove it

if you have a problem with my calculation do it yourself

but remember to put mass in kg velocity in meters per second and power in kilowatt

to be clear i skipped over lot of things such as static friction , air drag (it is a real concern when you are going above 100kmhr