Spose its possible to have an a15 in a Praire. Cant really discount it till you see the block number to confirm, or post a few pics up and it will be identified pretty quickly. Hec if its OHC then its E15. Still, im pretty sure that the prarie didnt come with an a15, and looking at wikipedia, the first generation came with an E15 amongst others.
Nissan Prarie Wikipedia



You could use an E15 in your car as they two engines (e15/A15) bolt up close enough to the A-series gearbox. There are a few issues such as disturbuters and oil coolers getting in the way, but thats all manageable.

I believe Matty1k? from Adelade has stuffed an e15 into a 1000, with only a few mods, so it can be done.