I wasn't talking about the 900 either, I was suggesting that your statement about jap bikes might not apply to the SOHC4s we all hold dear or that there are some jap bikes where cost isn't the only determining factor for repair.
For the 900 in question I would look at the cost to repair versus what the bike would sell for when repaired. A good running 900 Custom in my area is worth $2000-2500 unless it has a Hondaline fairing and luggage. I have seen dressed 900s go for up to $3600. If the bike could be bought as-is for $400 and an engine for another $400 (Two Wheel Heaven, a nearby yard has 2 of them for $325 each) You would have $800 invested. Let's say the engine can be changed in 10 hours (more likely 4-6 if you are skilled) and your labor is worth $40 per hour then your labor cost would be another $400 for a grand total investment of $1200. That gives you an $800 clear profit if you sold at $2000, and a $1300 clear profit at $2500. That's a possible return on investment of 60% to 100% after you get paid $40/hr to fix the bike. It's a gamble, for sure but it's a gamble I would take.