Crosman has some parts available online. See the link below for all the Crosman rifle parts manuals and owners manuals. You can order parts from Crosman.
The CROSMAN 760 Powermaster started production in 1966 then went through 8 different changes and ended in 1991
There are several versions of this model. Here is the web page for all of Crosman air guns. Owners Manuals and Schematics. See the link below.
It all depends on the current condition of the rifle. You have not given description so it is not possible to give an estimate of value
The Remington Airmaster is rated at 725 FPS (Feet Per Second). The Crosman 760 Powermaster is rated at 595 FPS and the 760P is rated at 600 FPS. 10 pumps each.
It's usually just in front of the trigger guard. It should be a 9 digit number.
You have two choices. First: Crosman has the parts manuals online along with the owners manuals. (See the links below) there are several versions of the 760 so look at each one to find yours. Then contact Crosman for the parts you need if they are available. (Customer contact.) Second: If you have no luck with that, Crosman offers a free service to locate shops that repair old Crosman products. So see the 3 links below.
There are two CROSMAN Model POWERMASTERS. In Excellent condition they are worth as follows: (This means the rifle is also in Excellent looking condition not just working condition) Model 760 POWERMASTER made in several variations from 1964 to 1991 around $20 to $25 Model 760XL POWERMASTER made from 1978-1980 around $30 to $35. If the rifle is in used looking condition the price drops considerably.
The Crosman 760 doesn't shoot Bullets it shoots BB's or .177 caliber Pellets.
They are inexpensive and not worth more than $5. However It's just fine for the 760. A good scope will cost you more than $100. To keep the cost down Crosman uses inexpensive scopes on the 760. Personally I would not change it out.
There are several versions of the 760. See the link below for all of the crosman models with dates and manuals.
Crosman has made 5 or 6 versions of the 760 and has parts available (See the link below for the older 760 parts list. ) I would contact Crosman Customer service to order the parts (see the Link below)