I would be curious about what the backboard electronics consisted of and if you were able to use stuff like a normal computer motherboard for part of it and what the roms were installed on. The other thing is making the ramps, if you made them out of metal then no problems there, if you were looking at the plastic ramps, then you would probably spend a good chunk of money having them custom made (unless you had your own molding machines :D ). I would say the biggest obstacle would be the money involved, including ordering pinball specific items (flippers, pegs,plastic ramps, etc). The rest you should be able to fabricate (cabinet, playfield, metal ramps).
MMM I just thought of something, I wonder if there was a pinball machine that used metal flippers? Wonder what the "feel" would be like? :D