I think I've done a lap somewhere between a 8:20-8:30 with a 125 kart.
But seriously I think you should try one of the old Volkswagens or the 2CV.
You could always go backwards - in a Swimmwagen.
UPDATE:
I done it in the Subaru 360, full 200kg ballast with a huge 16bhp. It took me........................................................Wait for it...................................................16.46.XXX
And you managed to get up the hills???
Try full weather, 100% water, rain, at night, no lights, comfort hard, tire wear on, any sub 20HP cars, full ballast, no oil change, high miles, shortest gearing, let me know the time