Marantz 8b power cord replacement

Any quality 1.0 gauge wire should do I believe that is 18 AWG.

If the power cord is 16 gauge then you could buy one of these and just cut off the IEC connector.

It appears the Marantz 8B uses a 3.5 amp slow blow AC line fuse. Just a guess the full load amps of the Marantz amp is around 2.8 amps. #16 gauge copper wire is good for 10 amps.
https://www.ampslab.com/SCHEMATICS/Marantz8B.jpg

The cord has a non- polarized 2 blade plug which matches the plug on the Marantz 8Bamp and the tuner receptacle outlet.
https://www.amazon.com/Cable-Matters-2-Pack-Non-Polarized-Power/dp/B0153T1MHM?th=1

Jim

One additional source you could check with is VAC.  VAC and Kevin Hayes manufactured the Marantz reissues in the early 90’s and he could be familiar with the original specification.