Magneton gets two-shot by any ground type pokemon worth training, and one-shot by a high-power Double Kick. Steel may be supereffective against Rock, but you can grab a Sandslash or even a Nidoking/queen relatively easily by that point in the game-- which are both capable of shrugging off the one Flash Cannon that the Magneton will be able to fire off with ease.
Electric types just flat out don't have a lot of options when it comes to fighting ground types, particularly the Kanto ones. You could give Lance a Lanturn, but that's liable to get one-shot before it can even do anything. Ice and grass attacks? Hella rare on the electric crew, outside of Rotom, the electric legendaries and Lanturn.
... One option might be Electabuzz, which IIRC has access to Ice Punch, and is pretty fast.