What are you hoping to achieve here? Single page apps are a disaster, particularly on mobile devices. What don't you like about the current site? Can you not access the content you want? Is there some device you have on which it doesn't work? Or are you just saying that you subjectively dislike it because it doesn't use your chosen techology architecture?
These are just a some ideas, I have worked with and delivered a fair few SPA websites without issue. well... without too many issues

.
Maybe not a SPA site, I am just thinking maybe a new version built with tech that more people develop with which *may* generate more support in terms of having coders with free time who use this site and are willing to give up some free time.
That would be my goal in choice of tech. If SMF meets the criteria then go with it, if it does not then go with something. But I guess at the same time, it needs to be inline with the Admins skill set.
Ofc, I dont know enough about the current project to know, but I felt it was worth putting the idea out there.
My angle is purely from a get more help with the code even if that means changing the tech rather than saying change tech for the sake of changing it. Maybe even have a backlog? that punters who contribute the most can add items to.
UKP is awesome, yes it does feel dated and somethings could be improved but it does the job well enough.
I probably have over estimated the amount of devs on this site with free time

I know I have very little free time and I find it hard to contribute. I don't think it would look good on my time-sheet. "Doing dev work for UK Punting" my boss would be asking some awkward questions.