It wouldn't have been the fitting of the Malossi kit. That's literally just changing the barrel, piston, and main jet in the carb. Unless you had the engine ported too? In which case the engine would have been taken apart.
Sounds to me that your clutch is slipping. When you push the kickstart down, it connects to the gearbox, which turns the clutch, which is on the left side of the crankshaft. If the cork on the clutch plates are worn, they will slip against the metal plates, rather than gripping, and your crankshaft won't turn.
Using the electric starter turns the flywheel, which is directly connected to the crankshaft. That's why it starts ok using that.
That makes sense, any other signs of slipping clutch? Sometimes I have trouble going into first, it's a bit of a wrench. I've been taking off from the lights in second more often than not. Is that related? I didn't have the engine ported BTW. Cheers