One of the best inexpensive air pistols for the shooter and the tinkerer is the Crosman 2240. The basic 2240 is a .22 caliber, CO2 powered, bolt action pistol with adjustable sights, suitable for plinking, target, or pest control at short range against small birds, but the same basic gun comes in a wide range of variants, and a lot of customization parts are available from Crosman and aftermarket suppliers. Crosman offers a steel breech assembly with grooves for scope mounting for only $29.95, hand grips, shoulder stocks, different barrels, etc.
One of the more complete suppliers of custom parts is SD Custom Design, who have stocks, grips, muzzle weights and adapters, moderaators, bulk CO2 adapters, trigger springs, breech assemblies… and more. They’re in the UK, but will ship internationally. In the US, Mac 1 airguns have been customizing Crosmans for years (I used to have a Mac modified Mark-II). Mac supplies bulk CO2 kits, an accuracy prep kit (for only $25), a steel breech kit, and longer barrels.
Airgunsmith has custom triggers, sears, power adjusters, springs and folding stocks. RB Grips has grips and stocks for the Crosman and many other guns. No doubt there are a probably a lot more suppliers I have yet to find.