The 1800ES' were the least powerful of the series, as Volvo switched to a low-compression 'F' head and introduced emissions control equipment beginning with 1972 model year vehicles.

If your ES is now smog exempt, you could swap in an 'E' head from a '70-'71 1800E, which would theoretically yield an increase of about 18 horsepower. Supposing that there are no other differences between the B20E and the B20F, which could be a totally erroneous assumption on my part.

Beyond that, there are a plethora of aftermarket performance parts for your engine. Check with IPD (www.ipdusa.com) and RPR (www.rprusa.com).

Regarding your query as to what engines are interchangeable, any OHV red block motor will mate perfectly with your existing drive train. That includes the B16, B18, and B20. The B16 and B18 are, of course, smaller engines and would be a step backwards in terms of power and economy. None of the OHC engines (B21, B23, B230) will work.