No legendary can lay an egg except for manaphy which gives an egg to phione and you can not get manaphy or phione in heartgold
there are many legendary Pokemon in Pokemon heartgold, but the best is HO-OH
I put a link to a guide all about legendary pokemon in HeartGold in the related links.
In HeartGold you can catch Kyogre.
Articuno
mew
I don't think you can refight any legendary Pokemon except the wandering dogs.
no:(
migrate and trade
It is absolutely impossible to get Legendary Pokemon eggs.
A Togepi.
Kyogre is only available for Pokemon: Sapphire on the GBA. Ho-Oh is the legendary pokemon available to you in HeartGold.
yes at supercheats.com/pokemon heartgold action replay codes