Hi Bob,
I would suggest you locate an original Whizzer frame or one of the special frames made by Schwinn, Roadmaster, or Huffy. The most common is the Schwinn WZ/S4, and is easier to find replacement parts.
The original Schwinn WZ [later called the S4] has a heavy duty frame, larger spokes, a heavier fork spring, welded brake arm connection crimped frame and modified rear fender. All special frames were painted Whizzer colors, maroon & cream. There are a few exceptions with the 1951 and 1954 Schwinn "Specials" and most were bright red.
Another bike to consider is the Whizzer Pacemaker [IMO one of the best motorbikes ever made]. The original 1948 Pacemaker isn't my favorite, however the later 1948, 1949, 1950, 1951, and 1952 are. The differences are many, however the seat mounting position on the later versions is by far the best setup, as it is much larger, much lower, and very adjustable. The first Pacemakers used a seat mounted on the post just like a stock bicycle, and set very high when riding.
If you elect to use the 24" Pacemaker frame, the following information may be of importance:
Early 1948 "J" motor with rear coaster brake & loop frame.
Late 1948 & 1949 "J" motor with rear coaster brake & loop frame.
1950 "300" motor, 5" rear Whizzer brake, & loop frame.
1951 "300" motor, 5" rear Whizzer brake [no longer a loop frame].
1952 "700" motor, 5" rear Whizzer brake.
Other rare bikes to consider is the 24" Schwinn S10, as it was made to fit the Whizzer motor and is low and long.
You could also consider the 20" Sportsman [kick start], and the super rare 1951 24" Whizzer Ambassador [I had 2 of them as a youth in Ohio].
I have a large collection of vintage and new edition Whizzer parts if needed.
Have fun,