Rough idle turned to crank then ka-thunk

Have you DONE anything to it immediately previous? EG the dist looks might clean is it new? Did you change the timing?

If you have not moved the timing I would attempt to check it. Sometimes you can pull the coil wire and leave it "lying" near the top tower so it has to jump spark. This sometimes will relieve the kick so you can check timing.

If you cannot do that attempt to check static time. Bump the starter and hand crank it so the timing marks are about where the timing should be. Examine the rotor tip does it point to either no1 or 1/2 turn away? And examine, the pickup coil core should be aligned with the reluctor tip

You may have broken a distributor drive gear or slipped a timing chain

I don't know about "all" but many slants use a plastic distributor gear