Serious suggestions for reducing understeer??

[Island kart]

Just built my first metal kart after my previous wooden kart build, using my knowledge from my A level physics and inspiration from my old mini cooper I am a little afraid I may have gone too light (somewhere under 50 lbs including the engine), I put the driver and the engine right over the back axle and i know thats a bad idea but the engine's got some kick and i needed the traction. Well anyway the kart is too crazy (great fun, but crazy) it understeers and the front wheels lift off and there's a lot of countersteering on pain of death that has to be done. Anyone got any reasonable ideas for getting rid of it, cant move the engine, cant move the seat so i was thinking of putting in a floor at the front made from plywood to weigh it down a bit. Think the kart will be fine on tarmac, just on the grass its a crazy beast.