Add a little "power" to my DD engine

[Pettersen]

So i got a stock S4 N/A engine i use to DD while i'm building a Semi PP N/A engine.

And since i'm fairly new to rotaries i wonder if there is anything i can do with the S4 engine that would gain me some extra power without being expensive, since i need to put all money in to the Semi PP build.

Already have uprated exhaust,TB mod and a previous owner changed out the filterbox with an open filter, anything else i could do? My only idea is to build a proper CAI, since the setup now probably isn't optimal.