I would look at the serial number on the engine case. Check it against the manufactured engine numbers. Go to
http://www.dgi.net/914/ and check out the Tech specs on the 914. It says a 1974 should have engine serial numbers like so
1.8l engine EC 0000001 - EC 0037551
2.0l engine GA 0006766 - GA 0015021
I hope this helps