Any small engineering workshop will be able to whip you up a steel spindle housing to replace the original.
If you can still get the shaft & pulleys , all the rest is shelf hard wear.
There is a mob down here who makes steel replacement housings to replace the 3 leg AYP ones that brake if they run over a fart.
If that is not an option, go to the Rotary, Stens & Oregon web sites and search for blade spindles or blade mandrels or both.
In the "More information " or "detail" window will be the dimensions of all of them so it is a matter of finding one that either fits or can be made to fit.
MTD started to cull support of older Cubs a while back & one of the ways they do this is to no longer list or keep the blade spindle assemblies.
Also check ebay as in the case of Cub, a couple of ebay vendors obviously bought the old inventory & were selling the old 2000 series spindle assemblies cheaper than I could get a bare spindle for