The 45 indicates the bore size (45 mm).
Stoke could be 35 mm as in a 3.4 ci engine (Model 116), or 32 mm as in a 3.1 ci engine (Model 112, 113 or 114).
You might be able to measure the stroke straight down through the spark plug hole with the end shaft of a dial caliper. It's something around 50 cc.

Tom