I am very knowledgeable about the 175/250 Aermacchi Harleys both inside and out. I have both a 1976 SS250 and a MX250 in a very lightened 76 SS chassis (street tracker) Have a big honking stockpile of parts to make at least two more complete bikes. One project in the distant future is to put a Cagiva WMX 500 two stroke single in one of these frames muahaha. Once I nail the jetting on the MX250, it will be a rocket enough...
If you have any questions just ask!
I would suggest getting a better bike with a title, and using the 74 for parts or profit. 74 and 75 had alot of "early" parts, them being the first runs years. For instance the two piece kickstarter, the Zener diode rectifier ( which is missing on yours), the lack of rubber insulators on the side of the cylinder, plus many other minor and noramally unoticable internal updates and changes. that are different from the 76-on models. The 74 also has a much weaker transmission, the later Cima trandmissions are much more robust and well-mchined.
Best source for NEW or NOS parts is Moto Italia (Leslie is a FANTASTIC guy to deal with) in California, or Charleston Custom cycle in Illinois. (which I have limited experience with)
For good used parts, Ebay is your friend.
Overall these bikes are pretty darn reliable. Dansi CDI Ignition, Mikuni Oil Injection, and Dellorto Carburetion. Keep you oil pump dialed in and airbox sealed, and they are pretty easy starting, well running, light handling machines. The only real mod I would suggest for the electrics is to replace the junk Dansi Regulator/Rectifier with a Japanese Nippondenso/Shindengen RR from a UJM. The 175 and 250's are magneto fed CDI, so they will run without a battery. But if you want to have a reliable lighting system, opt for the jap RR retrofit.