Zerostart 310-0105 is a part # for full kit
Zerostart & temro catalog does not show part # for just the power cord ( that I could find )

I would probably cut off the old plug & put a replacement plug on the old cable. Just mark which wires go where.

This shows one guy doing something similar:

How To Replace The Plug On An Extension Cord Or Power Cord

Appears to have been originally made in Canada but they closed the plant in 2012

http://www.cbc.ca/news/canada/manitoba/winnipeg-block-heater-plant-shutting-down-1.1178759

This link talks about heater cords & has a link for ordering a cable that might work: (3600008)

https://mbworld.org/forums/diesel-forum/603521-where-my-block-heater.html