All I can do is guess, but my guess is that there ARE numbers on jdm vehicles/engines.

You might try calling some local shops that import engines for swaps.
Even junkyards that are importing regular used engines would probably be able to tell you if there are id numbers on the engines.

If you're able to inspect the engine before you buy, look to see if it looks like numbers or a plate's been removed or drilled out. If it looks suspicious, use your head and walk away.

I hope this helps.