"I would if I could." is almost idiomatic, and as such it is pretty meaningless as an example of correct grammar (idioms can be pretty loose as far a grammar goes). "I would if I can." is a little edgy grammatically but is apparently possible, since "would" is a modal and is not strictly the past tense of "will." For example, you can say: "I would if I'm able to." Here you have a present tense