Tough for me to comment on what emulators are easy or difficult. I've been using DOS since it was relevant and so I have a slightly warped sense of easy of use for it. I'll try and get some perspective and add links accordingly.
As for your question about remaking a game you already have as a .COM file, that's absolutely okay! I might suggest creating a separate page for the .COM version for the sake of the jam tho, so as to keep it clear that it conforms to the restrictions set by the jam.