[Yosemite Sam]

You gotta pull the trans-axle out (I don't know if you can get the shaft back in from the front) if you take the engine out instead.

Then knock the pin out of the drive shaft and put the fan on.

There should be a shroud screwed to the tunnel that the fan goes in.

123's came with a metal fan set in rubber, I would guess that a new style plastic fan would work if it is the correct diameter. Be careful putting it in (do it on a warm day) the plastic fans are pretty brittle.

Now that I think about it, the plastic fans may be a little larger in diameter...