How do I change Go Kart tires And how do I place or replace a Go Kart Engine Thanks?

There should be 4 bolts on the underside of the engine mounts used to release the engine from the chassis, you then need to unbolt the remaining half of the engine mounts from the old engine and fit them to the new one (unless of course you have new mounts too) and re fit to the chassis ensuring there is not too much play in the chain. as for the tyres i'd try to get your hands on a tyre changing tool available from ebay, zoomkarts etc, if not just let the air out and persevere with a thin strong metal bar.