For me, the 440 "A" and "D" models would entail replacing the rear wheel for an 18 inch wheel with a disc brake. The frame is unsuitable for a flat seat. The gas tank is too small.
The 440 "C" model would be ideal in stock form but they seem extremely rare in the US. An alternative, for me, might be going with the "B" model but I'd want to ditch both wheels for mags and have a front disc.
The 550GPZ has beefier front forks, 18 inch rear wheel with a disc brake, a bigger gas tank and the seat and handlebars I want. It has everything I want EXCEPT a 2 cylinder engine.
(Since I was broaching the idea of switching to the 440 engine in the 550 frame I just, as an aside, mentioned the 750 Twin engine in case anyone here had done such a thing it might provide additional ideas. We can just skip that whole idea).
When I compare what I'd like to switch over TO a 440 I just started wondering if it made more sense to get the chassis I want and just replace the engine?
Gas mileage is important and the 440 would be much more appropriate today. My very first motorcycle was a brand new 1980 KZ440A1 which I put over 33,000 miles in a little less than 3 years. I drove it from FL to WY and NM to FL without any problems and a few other 800 mile trips. I kept track of every drop of gasoline that went into that bike and most tank loads were 60mpg+.