Grant;
To try to answer your initial question.... the rule of thumb in trying to equate a 2 stroke to a 4 stroke is you size 1 up from the 2 stroke! for a 45 2s look in the 60-75 4s, 60 2s is 90-100 4s, and 90 2s is 120 4s. This is a rule if you want equal power. Of course most of us want more power....
Now for power vs size, the YS-63 will give you the same power as most 90 4 strokes. So there's alot of bang for the buck there. If you are just starting in 4 strokes you might want to get an OS. They are very user friendly for first timers. Saito's and Magnums take a little more breakin time and tlc. While the YS, you just have to be a little more meticulous in your setup.
Good luck and remember to just have fun!