When taking my junk-picked 70s-era lawnmower apart, besides finding out that this motor was made the same year I was, I realized that the exhaust threads are a standard pitch 1/2 inch pipe thread!
The parts you will need are:
A short 1/2 inch nipple (short pipe, threaded on both ends)
A 1/2 inch x 1 inch hex bushing (female 1/2 inch on one end, male 1 inch on the other)
A 1 inch 90 degree elbow
A length of 1 inch pipe
A 1 inch cap
A drill and small drill bit(s)
Cutting oil for drilling (WD-40 or even motor oil will probably work)
(optional) Pieces of fiberglass tape or cloth for sound-deadening material.
PLEASE use black gas piping, NOT galvanized pipe. Galvanized pipe is coated inside and out in zinc. Zinc heated to very high temperatures can offgas and make you feel very sick. There is some debate as to whether zinc exposure leads to long term health effects, but I have gotten some stern warnings against welding zinc, so have used only black gas piping for this project.
You could also probably do this project with aluminum or threaded copper pipe, but for safety and affordability, I went with the steel. You probably wouldn't want to use soldered copper, in case the solder were to melt and cause trouble.