Disclaimer: I am not a machine restoration expert; nor do I play one on TV.
However, some things do come to mind:
First, if you aren't already subscribed to Kieth Rucker's YouTube channel; you should check it out. Amongst other things, he does machine rebuilding. Ox Tools is another good one, too...
https://www.youtube.com/user/ksruckerowwm
https://www.youtube.com/user/oxtoolco
If you don't already have it, get some high-spot bluing compound.
Supposedly, float glass is really flat; and sometimes you can find granite tile that's very flat also (although, when you are talking about one that's tile-sized; a proper granite surface plate isn't
that expensive). Take three pieces and compare them against each other with the bluing compound - the only contact surface that can be common between three pieces like that is a flat plane. Not only can you use this principal to verify the plate's flatness; but you can also make them more flat by putting lapping compound on them and alternately rubbing them against each other (1 against 2, 1 against 3, and then 2 against 3). It is labor intensive; but it does work. Come to think of it; I might even get a small gear motor (like perhaps a windshield wiper motor off of a truck, er, lorry) to do the rubbing for me.