Based on the diagram I see no reason a cheap modern relay could not be used. The wire sizes marked on the diagram are 14 and 18 gauge wire, so they aren't carrying a lot of current.
My quick google search for 'Bosch rely' turned up a 30 amp for three bucks. I didn't think of Hela, but clearly something designed for continuous duty would be the ideal.
I'd sure try that before spending $100 on a used relay.
