The bike is certainly rideable with the crack as-is. I had a similar crack and asked about it on the framebuilders mailing list about 2.5 years ago. The almost-unanimous opinion is to just drill and polish a stop-hole and use a very-long seatpost, maybe a 400mm MTB seatpost, because the seatpost forms the butting at the top of the seat tube.
Here is my crack (and the thread) for reference :
http://search.bikelist.org/
- Don Gillies
San Diego, CA, USA