Yes, they are readily available from all the big component suppliers, for around £1
You can also use the PIC12F675, that a twin to the 629, it just has an extra analog input capability.
You need a separate programmer. A PICKIT3 is possibly the cheapest approach, you can get them on ebay for 10- or so.
The program is written in PIC assembly language and uses the execution times of the instructions as delay times, in places.
You would need to learn the assembly language of any other CPU family and re-create the functions and timing to get it to work.
It also means you cannot simply use any other PIC that runs at a different speed or has different instruction timing, without re-writing the program; it's built around those PIC12 devices.
There are various other decoder programs available using different PICs, eg. such as the MERG ones.
Edit - example programmer on ebay:
**broken link removed**