Ensembl protein identifiers from different assemblies
Those IDs are elderly! Ensembl 54 was 2009!

I would recommend using BioMart combined with the ID history converter. The ID history converter will convert old IDs to new, and BioMart will convert ENSPs to ENSGs. You can either use the ID history converter with the ENSPs first, then the current BioMart to get the ENSGs. Or you can use Ensembl 54 BioMart to convert the old ENSPs to old ENSGs, then update the ENSGs with the ID history converter. I would probably do both and compare results.