Motor swaps take you straight beyond Street Prepared (except for some rare cases). Chances are you'll land in Street modified.
Here's a link to the current Street Modified class rules. The rule of thumb is that if it doesn't expressly say you can do something, then you can't do it. However, note that 18.1.A says "All Street Touring modifications are allowed", and 18.1.B says "All Street Prepared modifications are allowed", so if you can find it written in the SP, ST, or SM rulsets, then it's legal in Street Mod. Clear as mud?