One of the relays on my boomslang headlight harness has corroded and is showing signs of overheating.

Are these relay holders generic? Would this do the job properly? LINK TO FLYING SOCKET
The relay that is causing the problem has the code H5458 and it's 12V 40A I can't find one. Does anyone know an equivalent code or where I can look for one? I've ordered a tool for removing the terminals from the socket.

Yep, those relays are generic and the socket you linked to will work. There are two types of relay: A and B - you can figure out which you have by looking at the terminal numbers and comparing with the info here:

https://www.hagerty.com/articles-videos/articles/2017/08/29/understanding-relays-part-2

But if you're wiring the socket in yourself, just make sure the terminal numbers go to the same wires as before.
40A is more than adequate for a headlight circuit (which draws around 10A if you have 2 x 55W bulbs) but you can find a 40A relay from the same supplier as the socket - item 140201.