I'm trying to find a 13b to swap into my 1983 FB and i'm not sure where the best place to get one is i've been looking at a guy who is based in australia but i don't know how reputable he is. https://philsrotary.com here is his website. Have any of y'all heard of him? If he's not good where else would y'all recommend buying one?

 

I think you can still buy them new from mazda

 

post a want to buy ad here

https://www.rx7club.com/market/

 

I thought you could get a 13B-REW block from Mazda which is what was used in the 3rd gen, but I don't see one on Jim Ellis Mazda parts site. Mazda Parts - Jim Ellis OEM Mazda Parts Mazdatrix has them though. ENGINE 93-95 RX7 13B NEW - Mazdatrix. IMO it would be the best 13B to use but I don't know if anyone makes mounts to put it in an '83 Rx-7 so one would have to fab their own. Looks like Racing Beat makes a manifold to put weber carbs on it or one could use their manifold to put weber style throttle bodies on it.
Weber Intake Manifold 48/51IDA for 93-95 Turbo Engine - Racing Beat
OP didn't state what 13B he wanted but a 13B could mean many things as different versions were put in cars throughout the years. None in 1983 though. The only versions I'm aware are still available new are the above mentioned 13B-REW and the 13B-MSP from the Rx-8. Jim Ellis has it listed as available and Pro-Jay makes an intake/throttle body setup for it. 2004-2011 MAZDA 13B RX8-X260-ITB RACING INTAKE SYSTEM – Pro-jay.