The old SOHC4 swingarms are rather rubbery, and several aftermarket replacements have been made over the decades.
An alloy one made with large section legs should be stiffer and lighter, both are good.
The usual modification to a stock one puts a reinforcing hoop between the legs, and/or adds stiffening webs along them. Stiffer, but heavier. Being heavier will negatively affect suspension compliance. The affect will be minor, though: the hub/wheel/tire weight is much greater than the change at the swingram (and the swingarm weight is not all at the axle, weight near the pivot doesn't make much change to the unsprung weight).
So probably stiffening a stock swingarm, done with some appreciation of the engineering principles involved (many "back yard" mods add metal and weight where it won't do much- if anything - for stiffness), should be a good mod.
Needed? No. The frame and forks are quite flexible compared to a modern sportbike and adding sriffness to the swingarm won't improve handling appreciably. You can't turn a pig's ear into a silk purse so easily. And no, I don't think these bikes suck: I love them. I also appreciate that mid-70s bikes are not comparable to modern motorcycles in chassis or engine performance.
