Turbo?
 

I heard that the gt06 and gt25-550 are both small enough to put on a 250cc engine but I couldn’t get a response from Garrett Motion on this. They also have the gt20 series and 660. I don’t know which would be best because on the gt website they all list 1.4L minimum displacement.

I want to spent $150 or less and put it on a Boom BD-250-5

Here is a phessio turbo good for 23 to 100 hp and costs $109. I'd love to have a turbo on my Xpect but there's no way to tune for it.
https://shop.phessio.com/product/vz2...v=7516fd43adaa

It says 500cc - 1000cc engine for the phessio.

Yeah, I noticed that but so far it's the smallest I've found. There used to be a turbo kit for the grom, you may be able to find a used setup. There is/was a video on youtube of a guy with a turbo kawasaki 250 dirtbike, may be able to get info from him.

https://www.ratchetwerks.com/ninja250
Here is a ninja 250 using a copy of that phessio turbo.