could I rasp out the diagonals and have a good Robotron stick?
A better option would be to find a used 8-way on ebay, and swap out the bases on the two.
That way you get all new guts/stick/switches/etc...
My fear is that if you just ream the hole, it may not feel right.
I personally have been really lucky at finding NOS 8-ways.
They are out there, if you watch for them.
There are alot of good condition used ones too.