The Cushman Trackster Service Manual recommends Champion J2J (Discontinued)

Champion J2J Cross-References to:
Autolite = 353
Champion = J4C (825)
Bosch = W5EC
NGK = B7S

NGK says:
B = 14 mm
R = Resistor (Used to reduce radio noise or interference)
# = Heat Range (2 Hot - 10 Cold) (Looks like 7 is the right heat for the Trackster)
S = Standard 2.6 mm electrode
G = Racing use
V = Fine-wire electrode, gold-palladium

Heat Range
A hot spark plug has a much slower rate of heat transfer and is used to
avoid fouling where combustion chamber or cylinder head temperatures are
relatively low. A cold spark plug dissipates heat more rapidly in hot running
engines to reduce the risk of pre-ignition. The correct heat range is found
when the plug prevents fouling and does not contribute to pre-ignition or
pre-detonation.

Fine Wire Electrodes (V)
Spark plugs with small diameter center electrodes (.050") were originally
designed to improve starting and anti-fouling characteristics in small
two-stroke engines such as snowmobiles, chain saws, and dirt bikes.

In Theory:
Good NGK plug numbers would be:

Hotter	Suggested	Cooler
B6S	B7S	B8S
B6G	B7G	B8G
B6V	B7V	B8V
BR6S	BR7S	BR8S
BR6G	BR7G	BR8G
BR6V	BR7V	BR8V