Foam veneer wing repair

Dean, a wing-tip is easy! There isn't much loading stress applied to a tip, unless you hit it with the floor too often.
So, as you have cut the veneer away (you should have left it, if it wasn't damaged!), try gently warming the foam core with a hair dryer - NOT a paint stripper!! - and see if the crease will expand itself out. If it is right at the tip, within an inch or so, don't even bother with that! Just use your covering iron, and iron it straight(ish), then it's just a case of re-sticking the veneer, and covering it again.
If you have discarded the veneer, see if a piece of cornflake packet is the same thickness, and use that.
Bond the veneer/cornflake packet to the foam with pva glue, and tape/strap/tie knots in bits of string/use elastic bands and clamp it down, then leave overnight at least, to dry, making sure the wing hasn't twisted.
When it is definitely dry, it is just a case of re-covering with your preferred medium, heat-shrink film, tish, etc. If you use tish and dope, be warned, cellulose dope eats polystyrene, so if there is the tiniest gap in the join in the veneer, there will be a hole underneath it, after you have applied the dope!
p.s. Check your lateral balance when you have finished!

Repairing the root end of a panel is a very different matter.

Dean, looks to me like both the leading and trailing edges are broken, you're going to have to cut them out, about 2" should be Ok and glue in some new. Remember you must do 'scarf' joints.

Ah - you said the wing-tip!
As Klippy said, the leading and trailing edges look cracked, and need mending, because they form most of the strength of a foam wing. All the foam really does is hold the veneer in shape!






Regarding what to use as veneer, I did put - try a piece of cornflakes box, to see if that is the same thickness.