From http://www.kangkeusa.com
If you click the 'SK Engines' link, then the 'Specifications' button it takes you here:
http://www.kangkeusa.com/Engines/specs-sk.htm

The chart gives the inmpression that the 90 will turn a 13x8 prop around 10400 rpm.

If that's true, it seems like it should turn a 14x6 or 15x4 pretty well.

I do not have any sort of thrust calculator at work, but a 14x6 or 15x4 in the mid 10k range should put out quite a bit more thrust than a 13x4 at about the same rpm.