A couple of things. The insert- screws can get tight due to the heat cycling. Also, in some cutter designs, the screws are purposefully offset by ~0.002" from the centerline which crowds the insert into the machined pocket. But bottom line bends the screw on a micro scale.
I would do the typical Kroil soak for a few days. One could also put it into the oven at ~300F and soak it for an hour and see if that helps as well, before employing more radical techniques.
When that is all over it will be time to decipher the insert size used. Early cutters were really poor about denoting the type (e.g. AKPT) of insert to be used. But hopefully a MFG name is on the cutterhead, and armed with that and or the model number you can back-search the proper insert. Kennametal has .pdf drawings on their cutters one can get from customer service with the proper insert type noted.
And THEN you get to pick the GRADE of insert...which in any given carbide manufacturer's lineup takes a fair bit of study to determine which is the one you want. In general terms there are super hard brittle inserts which last a very long time, or softer carbides which wear out quicker but can take more abuse from interrupted cuts, etc.
