Is the main jet for the 45 a small round head or is it the large round? I was told that there are two kinds. Is that true?

 

The larger round are for the 45. I think there are two different large round. One is Mikuni and one is dyno jet. I'm sure someone will jump in here to confirm.

Also there is a chart that shows the difference in the two. Search for "jets".

 

tm45 comes with large round, stoack carb uses small round.
you can use the small round in place of large round - the hole size is the same if the jet # matches, and that's all the matters.
you cannot use large round on the stock carb, because they will not fit up into the sleeve that wraps the jet.