12A\'s actually started with the RX-2 and 13B\'s the RX-4. An RX-4/5 13B is a very good engine, but getting hard to find these days.
12AT is also a good option but I\'m guessing for your buget you\'ll be looking at a pre 1985 RX-7 12A on a Nikki carb. Try and get the series 3 12A if you can as it has larger centre plate inlet ports (tall port we call em)
Give the guys on exile-rotorsport.com a shout as they\'re a club dedicated to the older Rotary engines and conversions. They also sell engines within the club for a lot cheaper than some of the ebay chancers.
Good luck with the project.