Same as anything else.
Technically yes.
If there is source and the eventual project is within viable limits then it stands a chance of going quite quickly.
If there is source and the eventual project is on the edge of what is technically feasible it may take a bit longer.
There is also the mid range thing of the original game is a bit outdated so how much you bring it into line with modern standards, or at least newer standards (the old consoles have some nice strategy games, few very old consoles have mice or touchscreen input and it would be rather poor to not have that, though at the same time many games were adapted to work with lesser control schemes, or may indeed revolve around them -- see something like the original arcade rampart).
If there is no source the the game will have to be reverse engineered first. This can take a very long time depending upon the approach used, age and origin of the game, and how many resources you care to reuse from the base game (all those things on sites like
https://osgameclones.com/ where you have to provide the original games? There is a reason). Most fan games like this are not going to be open source from what I have seen.
In this case you also have the fan game problem -- companies are duty bound to smack down fan games when they become if not aware of them but they pass some terribly nebulous point where you can be said to have ignored them (and thus your trademark). I don't know how a lot of these PC games are still around but porting it to the Switch is all but inviting Nintendo's lawyers to get involved. This is such a problem that many capable coders and artists and whatever else will refuse to work on fan games in favour of original works, to say nothing of making your own original stuff being more fun anyway (if you are the kind of person that can be useful to a porting effort like this you probably have a thousand ideas of your own). I certainly encourage this mindset in people I might be teaching about ROM hacking and such. I typically view them as inherently worthless, never to get a community surrounding them, and at best can hope to shine very brightly before being smacked down into obscurity. Compare it to an out and out original effort like dwarf fortress.
Emulation, simulation and wrappers is another thing entirely (though wrappers also feeds on from the "provide the original game" thing above). I can't be bothered to look at that game but while it might take years to port say a GB era game (sticking with pokemon the disassembly work for it is fairly complete but still ongoing, and it has a lot of dedicated people working on it, it is also one of the very few games to ever have got that treatment) I can knock out an emulator in fairly short order (the bigger the power difference between the original and the easier it gets). For a modern PC game then that is a different matter. For a browser based thing then yeah.
