Ive used NOS for a long time on street cars. The kits for 4bangers is very safe. I dynoed an engine with NOS and the exhaust temps stayed the same. The big thing with NOS is make sure you keep your fuel system up in good working order. The import NOS kits can be set as low as 35hp which is plenty safe for these motors. With that small of hp you dont need to do anything with the timing or sparkplugs. If anyone wants more info give me an email and Ill tell you what you need to know.
Later,
Bill