Some of the hardest parts of the compiler are optimization and clear error handling/reporting. If you forego those - because you're testing against a codebase that is already free of things that break compilation and have no particular performance requirements for the generated code - it's a substantially simpler task.
Making a basic C compiler, without much error/warn detection and/or optimizations, is as a matter if fact no so difficult. In many Universities is a semester project for 2 to 3 students.
