If you can get hold of an old steel frame cruiser (like Cleveland Welding, or some such) for a decent price and no cracks anywhere, I'd grab that. My old 1948 JC Higgins is one of those.
Others on the forum will have more experience with aluminum or other alloy frames. I will let them chime in. They'll know more about it than I would.
I mostly trust steel because I can weld it if I have to. I haven't yet welded aluminum.
Ok um... Micargi makes some decent bikes, lots of members using them including me.
Micargi Mustang GTS
That's Dax F80-RT head
A few other bits and pieces on Murder
Not familiar at all with fourstroke kits. But as a general rule, try and find a steel frame, Allen is right, old American steel often passes muster for a long time. A vintage Schwinn frame (Chicago built) is incredibly strong. If you're interested, I've got a 1963 Schwinn American, with all original parts, sans tires and tubes, that I could be talked out of...for way more than it's worth...lol
If you want to make sure you've got engine room, the Micargi Stretch frames have it in spades. There are custom builders like Sportsman Flyer, you could look around for KCVale's builds, he does all kinds of cool stuff