Have you tried the "Wanted" section of Turbobricks yet? The Americans have many bits which they call surplus, but are very rare over here.
You could try asking around the marinas for any 1974 / 75 AQ170 (B30) engines they may be scrapping. But boat people do like to add zeros to their parts prices.
Depending on your budget, you may want to go to a custom flywheel - like a TTV Racing item (as Egads suggested) or John Vanlandingham (JVAB Imports, in the US), who makes custom "Supa Bitchin'" flywheels. The TTV one will be expensive, John's less so.
If your budget is tight, it might be cheaper changing to a 6-bolt crank and using a 6-bolt flywheel - of which there's one on eBay right now for about $196.89 AUD plus $78.62 postage.
ebay.com.au/itm/290958646586