Well, considering TLK isn't going to go with biology
the truth is if Simba had to depend on bugs alone he would have died of starvation. Simba probably could have learned to stalk and kill lesser animals in time such as rabbits, mice, birds, etc without having to rely on more honed instincts learned during cubhood. He would have had to supplemented his diet with meat. The thing is IRL some owners tried to feed their cat a total vegetarian diet (I know it's called an insectivore, but to a lion this is pretty much like being a vegetarian) nearly died. IMO, Kovu would have lost because Simba is an mature, adult male. He has size going for him and experience. We have no idea what other possible fights Simba has had to ensure the safety of the Pridelands. He could have to fight off other rogue males, for example. In TLG it is shown that Simba has more experience dealing with issues in the kingdom, so he's probably had more battle experience.
For those who say Simba looks "fat" in SP besides the obvious change in animation style, take into account Simba is older and has been on a meat diet as opposed to a insect/meat diet when he lived in the jungle. Simba would also have the pride behind him and the rest of the kingdom if Kovu did attack him. When Simba is fighting Scar we can see that Scar's mane does give him an advantage over Simba because he's more mature and thus the thickness of the mane. Keep in mind, wild lions rarely have the full, bushy mane seen in domesticated lions because of brush, fights, etc. Simba was tired in the fight because he had been injured earlier and he was attacked by multiple lionesses. As someone said, training is not akin to experience. As a teacher, you receive a lot of training but until you get in a classroom full of kids it's a different ballgame
The same with soldiers. Before they go to battle they have basic training and sometimes more specialized training, but ask any vet how different is it when an actual battle begins.
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