Nicely done tutorial on how Racket can be used to write new languages, with a syntax that significantly differs from S-expressions. Apparently, it's super easy.
because Lisp's fixnums are 30-bits and don't overflow.
That depends on implementation and even in the same implementation, 32-bit and 64-bit versions will have different fixnum sizes. The standard way of handling it is to just logand the results with (1- (ash 1 32)), and maybe sprinkle some the declarations and maybe some implementation-specific ones too. Some implementations have efficient MD5/SHA1 implementations which can be examined to see what declarations should be used for efficient compilation (if you don't care about that, then just don't use any and leave it to the compiler).