Hi,
I use the default/built in mikrotik vpn to access our network from home if required.It works well and I assumed it was pretty secure. Am I right?

Is it possible to set up 2 factor authentication with it for extra security?
Thanks.

AFAIK, it is not possible when using built-in authentication, however should be possible with some external RADIUS servers.

How difficult would it be to set up RADIUS servers for this from a technical standpoint?

Thanks, I'm curious how difficult that would be myself.