OpenRCT2 is open/free engine of Roller Coaster Tycoon 2 as OpenTTD for Transport Tycoon.
I found this by chance in Reddit....and it sounds great (I love this closed game )
In opposite of FreeRCT, this project takes the original compiled code and it is decompile. The result decompile code is very obfuscated but this is job of the developer...he is working in make the code more human readable and replace the m$-win parts by open source parts such as SDL calls.