Strange that you'd pass up on easy extra sales, its not like MorphOS or genuine Amigas are different platforms from OS4 from a development perspective.
Perhaps just having a 68k only binary would ease your burden as it will run the same on all three platforms.
The key here is "easy": as said above, it's not as easy as it seems - and you're probably thinking of just the port, but that's only the tip of the iceberg. Believe me, I could make a Linux port in no time, and that would be rather "interesting" sales-wise (BTW, actually I have already produced some builds in the past and I will also make a final one for testing purposes).
A MorphOS port would be probably just as straightforward as Linux' (or Windows' or MacOS'). An AROS port certainly is (I know because I ported to it BOH, which uses the same framework).
On the other hand, a Classic version would require an entire rewrite in assembly, hardware hitting, redrawing of graphics and re-tracking of music and, even with that, it would not be possible to achieve the same results - the retro look might be misleading, but there's no way AGA can cope with that resolution (401x401, 601x601 or 801x801, with the bigger resolutions looking better), 24-bit depth and 8-bit alphablending. Mind you, the idea is admittedly tempting to me (I just love to do that kind of things - I've been doing that for years*), but then it would just be a port, a much less pleasant activity than creating a totally different and new game - at that point, I'd go for this second option (and maybe in future I will... who knows... but, as said, for the moment being at the horizon there's a C64 game)
*Just to prove that I'm speaking seriously, here are some of the projects I released (maybe you've even heard of them):
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ESA: fancy development tool;
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Follia NBA: the very first game I made on Amiga, and then updated countless times (and every now and then I'm tempted to push it beyond);
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MEMO: a little game which is also meant to be a tribute to the Amiga and its games;
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MOM: complex project of a handheld console, including custom OS and applications - while the target is not Amiga (but the console itself), the only emulator of such console I ever wrote is for Amiga.
(And I proudly list them on
my mini-website.)