Belts:
The OEM belt is a Mitsuboshi 23100-148-7030 available from Honda Dealers for $60.00
The OEM belt length measured on the top side (outside diameter) is 41 inches (104.14 centimeters)
The top and sides of the OEM belt (width) each measures 19/32 inch
The bottom width of the OEM belt measures 11/32 inch

Dayco #17415 (also has 13A055 stamped on it) will work for $12.00 (it is a 41.5" belt, slightly larger than stock)

Spark Plugs:

1978 to 1981 (Gap .028")
Champion RL86C or L86C
NGK BPR-6HS
ND W20FPR

1982 to 1983 (Gap .030")
Champion RL87YC or L87YC
NGK BPR-6HS
ND W20FPR

Removing the Carb on a PA50