I used to ride a lot in Callahan State Park in Framingham. No O map exists that I'm aware of, but the
trail map is pretty accurate, and with the MassGIS resources out there it might be fairly easy to come up with a hefty base map.
There's also Great Brook Farm State Park in Carlisle, which of course has an O map, though the riding isn't very technical, from what I recall. Maybe a good place to start?
Cutler Park in Needham/Newton has some interesting riding, but might be the toughest of the three to get an appropriate map made. I always thought it would be fun (but not necessarily challenging) to orienteer there, with the swampiness and the plank bridges.
Added note: I've never done a MTB-O, and these parks seems kind of small for what I would assume one would need, but it's what came to mind.