can he activate to target himself???

even though he is no longer on the field??

the targetting is upon resolution, so why can't you use his ability when he gets attacked, and then the attacker re-readies??

Targets are chosen before costs are paid, and so he is still in play when you must choose a target defender you control. Assuming Wyatt Wingfoot is a defender, he can target himself. The attack will resolve with no defender, and so the attacker will reready at conclusion.

Before the attack resolves; Wyatt's payment power will be negated, since by the time it resolves, the target is no longer there. When the attack resolves, it would be as HeroComplex posted.

Wyatt Wingfoot, Keewazi Adventurer
Character, Fantastic Four, 1
1/1
Activate, return Wyatt Wingfoot to owner's hand ---> Target defender you control has reinforcement this attack.
Discard Wyatt Wingfoot---> Target Fantastic Four character gains +1 DEF this attack. Use this power only while Wyatt Wingfoot is in your hand.