There are chips available that can wake up an eta a bit. Here's one: E28 528e, E30 325e Turner Motorsport Conforti Performance Chip - Turner Motorsport

It's also worth noting that the M20 has a timing belt, so the first item of business would be to change it (and the water pump and tensioner). It is an interference engine.

Oh yes, and there used to be a caveat that hex-headed head bolts on M20s were at risk of breaking off and punching holes in the head casting as they bounced around loose in the valve train. BMW has male torx head bolts available to replace them. Maybe the service has been done on this car -- it's been a known issue for some time.