Anything 2 liters or over (the various combos are too numerous to list, but more is better!) is so much fun in these cars. As Stan (and I think someone else) said above, with ported 40x35 mm heads (Panchitos are the flavor of the month) cam/rocker combo with 250-255° @ .050" duration/ close to ½" lift, 9:1 compression, 1 1/2" exhaust for 2,000-2100 cc's and 1 5/8" for 2276, and 44IDF's or 45Del's is a really great combo. It will rev to 6,000 rpm with power, produce 140-160 hp, have fantastic bottom end/lower midrange torque ( where we do 95 or 97% of our driving), get really decent mileage (as long as you've taken the time to jet and tune it properly and you're not driving it like a raped ape from the moment the key is turned 'till it's shut off), last almost as long (again, driving style will be the major factor) and be almost as easy to take care of as a stock 1600. An engine like this can be driven anywhere, any time!
A couple of caveats- anything over 2 liters seems to need an extra cooler (preferably thermostatically controlled so the engine still warms up quickly and doesn't cause premature wear) in these cars and attention to airflow into the engine compartment will be required- these things are air (as well as oil) cooled, afterall , bigger engines ingest a lot of air and not getting it right means overheating and premature death. The aquarium air tube/water glass vacuum test and a remote thermometer in the engine compartment itself are very helpful and will tell you when you've got it right. In his technical writings Gene Berg tells of a kit car with no air intake into the engine compartment at all (his words) killing engines in as little as 4,000 miles!