Anyone know where to find these plugs that are peened in the crank halves? They look like frezz plugs? Thanks 

As the book describes, these plugs are installed to reduce the volume of the crankcase. It is a practice known as "stuffing". The crank is drilled here to balance with the rod and piston but if these holes were left open their volume would be added to the overall crankcase volume.

I am a firm believer in stuffing crankcases, especially down there in the crank wheels. What volume you have should be above the crank wheels in the vicinity of the transfers. Modern bikes use phenolic stuffers and welsh plugs in the big end pin. They go out of their way to do this, it is beneficial.

I've never seen the cup plugs sold as spare parts Yamaha used on some models to stuff the lightening/balancing holes on some models. Here is a link to the company that patented the Welch plug and has a lot of Welch and Cup plug sizes available in both English and Metric sizes. www.hubbardspring.com/product-category/cupped-plugs/

Or you could try the cork/epoxy or other methods used to fill balance holes over the decades... I'm sure YouTube or Google has info on them.

Stuffing really made sense on some of the early two strokes like the popular derivatives of the DKW RT125... BSA Bantam, Harley Hummer, etc. It REALLY made a difference on those bikes as they had excessively large open space around the crank with a result of very low "crankcase compression" ratios.

By the 1970's quality manufacturers like Yamaha pretty much optimized the crankcase compression ratio... generally leaning towards the "high" side by Yamaha for the porting and engine cooling technology of the era.