A helicoil works just fine for this application. No need to overcomplicate something so simple.
As for going deeper on the case. I would only attempt this on a drill press or mill that has either an accurate depth indicator, or a stop set. In theory the idea is sound. More threads = more area of thread engagement = stronger.
The main issue is, going too far will put the hole into the crank case and that will cause a vacuum leak.
Some cases have a lot more material to work with than others. That is one aspect of the 110cc engines that I do love, they are thick and chunky and have ton of deck material to work with.