Cargo Bikes

This one here seems to fit the bill? £50 tho, ffs...

https://www.wiggle.co.uk/kmc-e101-ept-si­ngle-speed-e-bike-chain

Doesn't need to be that expensive, but it should be 3x the price of the one currently on there.

The KMC Z1eHX Wide EPT looks appropriate to me, but I didn't look for longer than a minute. It's a question of whether the wide format fits up front where your chainring guard is. Probably will.

EDIT: I just noticed your bike is not a mid drive e-bike. You have much less to worry about. Still looks like it needs to be fixed up to be made pleasurable to ride but my whole concern about it being an e-bike with tons of torque is invalid.