Most likely your problem is with the key tumbler assembly part of ignition lock.

Unfortunately, you can't easily replace the tumbler assembly like the 122, 544 and pre-1970 1800 Volvos.

I was able to do it for the 1800E/ES ignition locks, but the manufacturer of the replacement tumbler assembly that I was using no longer makes them.

And AFAIK, that manufacturer never made a replacement tumbler for the early 140/160 ignition locks. I know they are different as I tried to install one of the ones that worked on the 1800E/ES lock and although I made it physically fit inside, the 140 ignition lock assembly, the tumbler assembly was not clocked correctly and the key would not turn all the way off.

So your best bet is to find a good used one or if you are lucky, a NOS (New Old Stock) lock.
--
Eric
Hi Performance Automotive Service (formerly OVO or Old Volvos Only)
Torrance, CA 90502
hiperformanceautoservice.com or oldvolvosonly.com