Welcome along. Looks a good project, with a bit of history and personal connection.
I suspect the original failure will be in the crankshaft assembly..... These early engines have poor oil supply to the drive side big end, and the narrow oil way across the crank becomes filled with an oily sludge and it all comes to a stop, sometimes with truly spectacular results. See the pictures posted under Great Mechanical Disasters. Well documented on the forum, attending to this is probably the most important aspect of any engine rebuild.
From the pictures the front brake torque arm looks as if it only has one hole at the fork end.... So a homespun part? Pistons look flat top, so maybe not original, OE were low compression. Count the teeth on rear sprocket... 49 teeth on sidecar bike, makes it under geared. 42 teeth on A10 Solo.
As for parts, new repro bits are fraught with problems as in many cases the quality control is poor. At least used parts fitted in the factory, and there is some satisfaction in giving a used component a new lease of life.
These early bikes use BSW threads into Alloy. Majority of other fasteners are BSCy ("Cycle Thread") also known as CEI, which though looking like it are NOT the same as UNF. Some BSF are also used, but as an aid there is a useful nut and bolt chart in the Forum Literature Section, cross referencing fixing part numbers with thread size.
Swarfy
