You are right that simplified reimplementations make good property tests, but in this case I'd go the other way around: generate an AST, render it a in test case-dependent way (adding whitespace as you said, but also parens etc), inject known faults for a fraction of test cases, and check that the parsed AST is equivalent to the original one or errored out if a fault was injected. Rendering a given AST is usually simpler than parsing it.