There are some simple changes you can make to the .xfm files in order to "create" different variants of the Spad, but you have to edit them by hand for each plane in the squadron or flight.

The simplest, but crudest type of change of course, is to change the top speed value. For instance, the current top speed of the Spad VII is 138 mph. (BTW this is too high IMO, for ANY variant of the Spad VII). Change this value from 138 to 119 for the 150 HP variant. Also, change the horsepower value from 180 to 150, of course. You can also fiddle with the speed values at different heights, but I'm not sure whether they have as much of an effect.

The main problem, of course, is that you have to do this all by hand for each aircraft for each mission. I don't know if you can use JJJ's mission editor to make the task easier, or not. I could probably come up with a mod to create a 150 HP variant, but it would still mostly be a 180 HP Spad with a few numbers changed around. OBD encrypted a lot of the FM files from WOFF 1 to WOFF UE, so not as many FM variables can be modded, unfortunately. Ah, well, c'est la vie, c'est la guerre!