You cannot get it before the elite four in Heart gold. This is because you need to take the old amber to pewter museum to revive it. If you want to use it in Kanto, it will be revived at level 20 and because you would have already beaten the elite four your Pokemon would be high level, so you would be able to get higher level stronger Pokemon like the red gyarados or tyranitar by this point.
Therefore it isn't as good as other Pokemon you would have/can get at this point and would probably be more trouble than it's worth to level up.
Hope this helps.