A 6502 would be ...... amazing ... and probably quite large.

Doing it "virtually" in software would be the next best thing, and I've started to play with the idea in Blender.

Blender's game / physic's engine might be able to do it, else I'll just "fake" it

http://knexcomputer.blogspot.com/

The K'NEX calculator stands over 10 feet tall, and can perform 4 bit addition and subtraction operations in about 30 seconds. The slowest part of the operation is the user entering the balls. From there the balls trickle down, computing the result of the operation, and then sending that through a 4 bit decoder, which flips a flag that tells the user the answer. Since it is 4 bit, we can add and subtract numbers from 0 to 15.