My '97 750's frame is a bit tighter than some of the earlier ones I think. I have a nominal 42mm tire on mine (39mm actual width, 36mm actual height), and I'm not sure that I could fit an actual 45mm tire in mine. If it was labeled 45mm, and actually measured smaller, then it might fit.

To be sure, measure your frame. You'll have three tight spots:

1. Fork crown,
2. Chain stay bridge area (area down where a kickstand is often mounted, just behind the bottom bracket), and
3. Seat stay bridge area (area around the rear brakes, where a reflector might be mounted).

Measure total width in these areas where you suspect the widest part of the tire bulge might be and measure height from the rim surface. In the end, tire clearance can be difficult to estimate/measure, so it sometimes is a bit of a guessing game.