Cutting down a fin is in my experience a pretty trivial thing, and works well, especially if you reshape the tip to be squared off, i.e. omit any nice pointy tip that is demanding to shape. That is, a new tip that looks more like
http://www.select-hydrofoils.com/en/.../srx-freerace/ (can be even more squared off!) than
http://www.select-hydrofoils.com/en/...o-series/ride/ .