Paul,
I have a venturi for the 25 FP, but I don't think that will work, since the FP is a higher performance engine. Someone here must have one they can measure for you. Was just leaving for the field, and will check back when I return.
My 25 FP-S venturi is .260" ID, and projects about .446" above the case opening, but I don't know if the 25 came with two venturi, like the 40 FP-S.
Also have a 35 S that should be pretty close, and will measure that for you when I get home, if you don't get an answer here first.
Bill