You can remove one coil from the factory spring.
That's how I would do it. You can Google "cutting a coil on an air rifle spring". They're several youtube videos available.
Take a look at this u tube video.
m.youtube.com/watch?v=Dk2Swlm3kCw
I ended up selling it and keeping my FWB124D as my only springer but now I am having 2nd thoughts on shooting the 124D too much and am looking at getting a springer for "shooting".
I liked the HW35E, and admit, for shooting, it was better than the 124D. In short, I regret selling it :-(
Right now it is between the HW35E and a TX200 in walnut.
Both Macarri & Vortek make springs for the HW3, you can contact them about power levels. Vortek makes their products to order so they should be able to help you.