A B-series will fit in there, but it would be like dropping a small-block Chevy engine into a cement truck and trying to pull it around.
Drop in an H23 if you want to boost later on down the line, or drop in an H22 if you want to stay n/a. An H23 engine should cost you around $1500 shipped, while an H22 will probably be around $2000 shipped. Figure an extra $200-$300 for parts related to the swap. You should be able to do an H23 for under 2 grand total, and an H22 for under $2500.
You can do either swap yourself, it's real easy. Check the reference section for crank hp numbers, and extrapolate from there.