I would believe that hacking an existing receiver would be a lot easier than building from a schematic. But, without a schematic of the existing receiver you are working in the dark. You would learn a lot if you have the time to draw a schematic of it (reverse engineer it).
A schematic of any receiver between 40MHz to maybe 200MHz would tell you generally what components to expect.
When changing a 'license free' device away from its original frequency, be sure to move it to another 'license free' frequency. If you start using a frequency belonging to someone else, they will be sure annoyed. If it belongs to law enforcement, the military, or aviation you could be in big trouble.
(If you use a ham radio frequency, we will track you down more quickly than the others would.)