That's bad luck about the Multi-Power. It's worth checking the MP linkage. Follow it from the hand lever to the point where it enters the transmission casing and operate it. It's quite possible that it may have seized or become bent.
Double check and clean all battery connections. Fully charge the battery and use a second battery if you can with heavy jumper leads. Attach the jumper earth clamp to the chassis (starter mounting bolts)to ensure the best possible connection. If you have a block heater you probably will not have a 'thermostart' in the inlet manifold. Might be worth checking. DO avoid using starting fluid (laughing gas) you may do more damage.