measure the width and pitch, it's almost certain to fall into a standard type i.e. 3/8, 1/2, 5/8"
https://en.wikipedia.org/wiki/Roller_chain
you can then cross reference to:
https://www.usarollerchain.com/Metric-Roller-Chain-Chart-s/4870.htm
then buy from
https://www.engineersmate.com/products/BRITISH-STANDARD-ROLLER-CHAIN/C106058
or
https://www.bearingboys.co.uk/British-Standard--1305-c
that's where I'm buying it from at the moment...
hope this helps, if you get stuck, ping me the width and pitch.
mark