While I am not familar with this particular kit, Most clear fin materials I've seen used in kit models is a Modified Acrylic. (Really NOT a good choice but much Cheaper then Polycarbonate.)
As such you can use a number of "Solvent Welding products to fix the problem but ALL will distroy the Clear surface wherever the solvent hits the plastic. I would not suggest using Testors tube type plastic cement, but would go with either Plaststructs Acrylic solvent, Acetone, MEK, or best Solvent welding agent is (MC) Methylene Chloride. All of these water thin solvents should be applied with a fine tip brush (Not the applicator in the bottle). Solvent is sparingly stroked on both broken surfaces three or four times without letting the solvent run down the edge being treated (NOT EASY). After application of three or four coatings quickly join the broken parts, Holding in contact for a minute or two. You can use little pieces of tape to secure the parts together but these tape pieces WILL LEAVE a noticable mark.
Set the model aside to completely dry (overnight) with NO stress on the joint at all. I generally use some form of motor mount stand to get the rocket vertical with no pressure on the fins.
If you need more info on "Working with Plastic" in the library section on the
www.Narhams.org website you'll find Tech-Tips. Look up Tech Tip-017 it's a 12pg in-depth study on what sticks to what and some good info on most of the Plastic we encounter in the hobby. There is also a additional Photo file that shows most of what is mentioned in the text.
As mentioned above: if you can't make a decent repair on the material you have I'd suggest removeing the broken and cracked fins and replaceing them with clear Polycarbonate (Lexan). These will not break, Unless flown in the dead of winter where the temperature is below freezing. Lexan and other polycarbonates are Highly Break Resistant but not Unbreakable at very low temperatures.
