What do think a highly experienced builder might do?
I think they would do a bit of research before accepting the job of rebuilding a 50 year old engine.
Try with in reason to get replacement parts, and if they can’t, they will tell you sorry parts are not available and not accept the work.
If what your dealing with is poor quality bearings, you will have to dig deeper to try to find a good set of bearings.
You could try contacting Toyota parts and see if new old stock is perhaps available or inquire if they were sourced from a outside vendor that may still have them, or see if there is a cross reference to another application that may be available.
You could also make inquiries on Toyota forums for leads.
Being it is a 50 year old engine, I can see where sourcing replacement components would be a serious issue.