Over a period of 10 years, there were 4 versions of the 045/056. First was 50mm/75cc, then 52mm/82cc, then 54mm/87cc, then 56mm/94cc. I have examples of the last 3, an 045, 045 Super, 056Mag. All in all, they are a pretty strong saw. Tons of torque, not so fast.
Likely, your saw is 82cc. Seems to be the most common size. At least, I have several spare pistons/barrels in that size, though I stole the rings to use on my 048. I'd beware if your saw has the early Bosch electronic ignition. They don't have a good reputation and like to quit unexpectedly or just not start. Problem is, parts are getting hard to find, especially coils. I'm actually looking for one of those to complete another 045 I have.
Also, this saw is hard on the lower front AV mount at the bottom of the wrap handlebar. Those always seem to be broken.
Chris B.