Often repairing or restoring older guns can be a challenge when there are parts that are no longer offered by the manufacturer.  In some cases the gun may even still be in manufacturer but the manufacturer does not offer the needed part for individual sale.  Depending on the geometry of the part it may be a candidate for molding.  Parts without internal voids or undercuts are prime candidates for molding.  Looking at the part imagine a parting line in the middle of the part.  If you can see how a mold could close and separate around the part it is likely a good candidate for a mold.  Silicon molds are the easiest to make and work with for resin cast parts.  Resin cast parts can usually replace any plastic pieces and in come cases metal parts also depending on the required strength and wear properties.  I prefer to use Mold Max 60 by Smooth-On. It is harder than most other silicon mold materials and is heat resistant. It can be used to cast pewter as well as resins. It will also last longer than softer molds providing more parts for the life of the mold.
So the first thing needed to get started is a mold box. The mold box will be used to contain the part and liquid silicon while it cures around the part. A 3D printer is handy for making mold boxes. The box should be  larger than the part and provide at least 1/2" clearance around the part.  I prefer closer to an inch.  That way there is plenty of room for registers in the mold.  I printed the mold box in the image below.  You'll notice it bolts together.  The mold box needs to be disassembled during the mold making process so keep that in mind.
The next thing we will need is some modeling clay. The clay should be placed in the bottom of the mold box and be thick enough that one half of the part can be pressed down into the clay. Once the clay is placed in the mold box push the part into the clay and use your fingers and any other handy tools (popsicle stick, etc..) to move the clay tight up against the edges of the part so that the mold parting lines will be clean.    Next take a hex head bolt and using the bolt head press it into the clay around the part.  I stay far enough away from the part such that the clay is not pushed into the part.  Also press indentions in the corners of the mold.   I use a combination of bolt and the end of a sharpie marker to make registers. These voids in the clay will serve as registers for aligning the mold halves when it comes time to pour.  You can also place small sticks (like cabob skewers) in the mold touching the part and sides of the box for pour and air vents.  I simply drill pour and air vents in the mold after the mold is cured.