Yes, I'm not overly impressed with the bottom end on the Ryobi either, but I am using one at present as a weedie ( and it's ok for that), I also have an old one with a broken rod, (acquired from my neighbour), he broke by letting it run too slowly and suddenly snapping the throttle open and the rod just folded!

Don't know if Stihl manufacture them for Ryobi, or visa versa, - not at all up to the usual Stihl standard.

The little Mac bottom end is certainly a strong looking unit - I would definitely go for that even if it is a bit short, don't know how that engine would go for power, and it's still got that big coil up front.

If you get a chance have a look at the McCulloch 28 or 32 weedies, they have a few good features ( and shortcomings too), they may require a little porting, and have aluminium rods (not all that impressive). but they look promising with their cantilever cranks which are quite long and are mounted in two bearings, the coil sits clear of the fins, and they also have model aircraft type mounting lugs.
Soon you are going to have a massive arsenal of potential aircraft motors!