I'm not sure if this is the sort of thing that isn't easily possible until the forums overhaul, but this seems to be quite a problem on MFGG. Many of the users that join MFGG are quite young and inexperienced with forums, so they are not familiar with the practice of necroposting and tend not to read dates on topics. Could there be a warning provided whenever a user tries to make a post in a topic where the previous post is over a month old, explaining what necroposting is and asking them to rethink their post?

This is a really good idea, but I'd make it at least 45 days or 2 months instead of just 1 considering the forums are quite slow these days.

I've seen this feature on another forum before, where a message above the reply box said "the last reply was over three weeks ago".

I like it, although I'd certainly push for a more generous time space than 3 weeks. Two months sounds ideal IMO.

Yeah, a heads-up for the user sounds good to me.
However, the user should still have the option to reply - some sites prohibit replies once a topic has been inactive for too long.

I would put this on the "nice to have" list. If a topic has been inactive for longer than a couple of months, and you tried to post in it, you'd get a message along these lines: "This topic has been inactive since April 2, 2016. Are you sure you meant to post here?"

Posting in old topics should not be prohibited, though. There are many valid reasons to post in an inactive topic, especially if it's a game or art topic.