Go with the ST or TTPRO engine, either will turn a 13 very very nicely for you, reaching your speed goal. Both of them are great with castor. Even the G.75 will turn a 13x8-9 with very impressive numbers. I am not an O.S. guy either.

I would rather have new stuff to build with the $$ I save on tiger type engines. But that is just me.

You will hit your goal with either of the more affordable engines with your boat under 7 pounds. I would however go with the .90 range engines turning the higher pitch 13's. I don't think a .60-70 will produce the thrust you need with any prop.