So, I've been wanting to do this for a while. but didn't want to pay $200 For the Atkins kit. Well in the end I didn't save much money due to having to pay shipping from other countries. I think I'm at $175 counting some pulleys I bought but couldn't use. Knowing what I now know I could probably do it for cheaper, depending on shipping from Japan. So here was my journey.
I've been running a 626 Alt. with the dual belt adapter so I had the alt pulley covered .In Feb. of 2022 I saw some 20B pulleys on Ebay from a guy in Russia. So I bought them mistakenly thinking they were the same as 13B-RE pulleys which someone on here used to do this. They are not. They are a 7-rib belt and the crank pulley is too large diameter as it hit the water pump pulley when both are installed. The 7- rib may have been doable if things lined up. So I gave up until a couple of months ago I saw a 13B-RE crank pulley on Ebay from England and bought it. I thought now I'm in business. Nope the alignment wasn't right with the 20B water pump pulley and the Alt. pulley. Alignment did look like it may be right between the Alt. and water pump though (at this point I still had the dual belt pulley on). I had a FD main pulley I had bought and then discovered that the holes didn't line up with the hub when spun around(I didn't know when I bought it the direction they mounted to the hub). I put it on as a test using two of the holes that would line up and it aligned just right with the water pump. So I put everything back together with a plan to redrill a hole in the FD pulley in the future as three of the four holes line up. As luck would have it my alternator went out last week so I enacted my plan while it was all apart.
First I drilled the hole in the FD pulley that was needed.

I didn't mention it before but the A/C pulley wouldn't fit due to the additional Air Pump pulley addition. So I took an old A/C pulley and cut the center out to use as a spacer for the P/S alignment since my A/C doesn't work anyway.

Then I painted them and added timing marks to the FD pulley. Also added dimples to help algn them during installation similar to the factory pulleys.


Final results.


There may be a slight misalignment of the Alt. pulley but I really can't say for sure. If so it just needs a thin spacer behind the Alt. pulley. To run A/C you'll need to remove that additional pulley for the air pump some how. I will eventually as there are marks on it from where the P/S belt makes some contact when it deflects on decel. I found this place in Japan that has the 20B water pump pulley. Buy Genuine Mazda NF0115131 Pulley for Mazda Eunos Cosmo. Prices, fast shipping, photos - Amayama
I don't know what shipping would be or if they are legit. Maybe someone on here has bought from them and can chime in. They also have the 13B-RE pulley so I may order one to see if it aligns eventually (if one never pops up on Ebay) so I can have the proper 5 rib pulley. If it aligns which I won't know until I have one in my possession. I haven't spun it past 5k or so. I'll update later after I've been to redline. Knowing what I now know cost should be used FD pulley $20 to $30 plus $45 for 20B pulley plus shipping from Japan if you are already running an FD or 626 alternator.

ETA: I think this is the belt one would need. More Information for GATES K050316 (rockauto.com)
I have a cheap Master Pro from O'Rielly's on it now but it used the same part number as the Gates it looks like.

 

So, a little update. Probably accumulated about 40 miles or so over the weekend max rpm of 5000 to 5500rpm. Had some black residue on the front of the alt. water pump and front cover. I'm guessing from the belt potentially being slightly misaligned. I cleaned it off before I left for work. Drove a total of about 40 miles. Ran 1st and 2nd gears to 8k rpm with no problem. Looked at it when I got home and no new residue. Just more marking on the water pump pulley from the P/S belt so I am going to have to remove that pulley that is for the air pump.
I also need to decide what belt I want to replace this cheap one with. Gates or Bando. I've had good luck with both. Been running Bando more lately as they are cheaper. 5PK805 is probably the belt I would need from Bando.

 

Good stuff.

I had an unknown brand serpentine belt kit on a used t2 motor I picked up recently and I got a lot of questions about it.

your solution seems more elegant, I might have to scour rx7club and eBay for those water pump and crank pulleys.

 

Thanks. Yeah, just have to keep a watch on Ebay, or roll the dice on ordering from Japan.