Personally I wouldn't worry too much about the 'morality', but if it really bothers you - just buy a copy from ebay (plenty for sale). The original coders or the company will not make any money from it, but then you have the original cartridge and it is allowed to make a copy from it for personal use (even if it is a game from Nintendo).

You cannot buy this game or rom from the company anymore so they abandoned it that is why it is called abandonware.

That doesn't mean it is legal to download and play roms from an old game, but most companies don't care. They have made some money from it and have moved on.

For personal use I don't see any objection to use abandoned software.

Selling a product with roms on it is a different story. Then you are making profit from someones work without permission and that is where I draw the line. YMMV.