Pailler encryption of small integers to 32-bit integers
Regardless whether input is small, $n$ must be large enough to avoid factorization. Next, $r$ must be sampled from a large space to avoid decryption by trial-and-error. Some crypto and big-numbers library (bouncycastle, openssl, crypto..) might be handy to implement such an algorithm. It would be safe to choose an implementation rather than write it from scratch.