just an added comment about 6061 -I used it to make parts for my modified Austin Healey --40 years ago -I made wheel hub adapters - brackets etc.. Now days, I regularly replace 6061 landing gear with 7075 T6 or 2024 T3.
6061 -commonly used for machining - will bend -will distort - over long intervals -that is - a box made from it - smacked on a corner will "rack" or shift into a new shape.
It is good stuff - but it's bending resistance is not all that high- I did not invent the stuff - I just use it.
If you want real stiffness-go to the aircraft grades - but for the case on the hand done engines - I would use it-but I expect it to do only what it can -
Castings will typically fracture if really smacked - but they also will bend - depends on the metal guy who decides what he really wants to pour -or inject and or forge -etc.,
Either material is plenty good enough for our use-but one is not "demonstrably better" than the other in all characteristics.
Production costs are the single biggest reason for choices here.
I know I dove my 80 ZDZinto the ground vertically from about 500 ft -clear up to the firewall!
The casting did not bend or break-
Perhaps a fluke--
the bearings again got flat spots - I would not expect then to survive -based on years of fixing others crashed engines of may types.