The motor if original, will be a 144 or 170.
Can you examine the block to find an embossed casting number beginning with a large "C"? Look below the exhaust manifold.
Is there a tag on the carby, that you can record for us?
The 170 has a stroke of almost 3 inches. The 144 has a stroke of 2½ inches. Although it would be angled, removing the first spark plug and poking a length of rigid wire through the plug's hole while turning the motor by hand, will determine which engine it is. Make sure the ignition is off!
You will find the 170 records over 4½" wire length in the bore this way, while a 144 will show under 4½". Remember to subtract the thread length of the spark plug hole in the head (about 5/8"), when calculating.
Regards, Adam.