Generator pulley 59 for a fan bolt om

[3W Hank]

I has a the older 6V generator ( round cut off regulator ) and a pulley that one can bolt a fan to.
I will replace the generator to the never 40’s style 6V and separate regulator. The one I bougt has a pulley with no bolt holes to a fan ( se pic )
I saw one engine that had the never generator but a pulley with boltholes for a fan ( se pic )
But axle diameter is not the same so how is this done.