I'm not sure if the 1600 engine has the same design as my 900 (others on here may know better) but if so, the crankshaft has "plain", or sleeve type main bearings. This means a plain shaft turning inside a closely fitted bearing bore. On most cars, there are replaceable bearing inserts that can be changed out when the crankshaft bearing journal wears undersize, or if the crank is ground undersize. If the bearing is starved for oil, the insert can weld itself to the crankshaft and "spin" inside its bearing bore. The connecting rods on these bikes use these type bearing inserts, and can also be "spun" due to lack of oil.
On the bike, however, the main bearing is part of the crankcase, and not an insert. It cant really "spin" however it can be quickly and completely destroyed from lack of lubrication. The crankcase would have to be replaced, as well as the crankshaft. The connecting rod bearings are almost certainly damaged, and the rods would also be suspect as they get lots of unusual forces applied to them when their bearings get tight. Get prices on at least the cases, crank and rod bearings, as well as all the gaskets and seals and everything else associated with a bottom end overhaul. I would look at the oil pump too, they don't like running without oil! Lots of labor involved, and you may well decide it's not worth it.
Ron in Cincinnati
Wish I didn't know now what I didn't know then.