The Neutron Filter is exactly like the MU3 circuit, but you will want to go with the hamamatsu P873-13 for your optoisolator similiar to the original.http://www.smallbearelec.com/Detail.bok?no=360
When i was cloning MU3's i actually copied and etched my circuit boards from the Original MU3 Board and just used the parts list of the neutron filter. the cost of parts, labor hinder selling them as it is cheaper to just buy a q tron, or repair a defective MU3. But i got very close to the originals and use my clones exclusively now. I could hack up a HAZ and insert the circuit from them no problem.
i spoke too soon, i looked over my notes and there are some differences in the neutron parts list vs. the original mu3. mostly the ceramic caps, the .1uf is a mylar in the original. the neutron uses a .1uf Tantalum. if anyone has any questions drop me a pm and i'll help the best i can.
as for mu3 clones, i do have one for sale, drop me a pm if interested.