After 10 years, my aircooled 82 Westy failed to make it home under its own power for the first time today.  I haven't driven the van in about 2 weeks and when I left home, a sticky lifter was making noise (as usual when the van sits for a while).  After about 10 minutes of driving in very light traffic in town (about 3 miles), the noise got louder and a more pronounced clunking noise was present and then it started to lose power.  I turned back for home, but the condition deterioriated with a sort of backfiring sound and some metallic clunking. It sounded like there was no exhaust on it with a clunking . I drove about 2 miles and when I got stuck behind a slow car and had to stop, the van died.  I restarted it but had a hard time keeping running and finally I pulled over.  I did a visual check and saw nothing - vacuum lines look OK, oil level OK, no oil leaks, no smoke, plugs and wires were fine, it had spark, no fuel line problems, etc. The sound seems to be coming from passenger side of engine towards front of the van. I restarted it but the clanking was still there and the engine would jerk violently once each time it made the backfire-like noise (it wasn't backfiring) so I called and it got a ride home on a flatbed. 

I've looked through the archives and overheating doesn't seem likely on a 50� day and only 3 miles (downhill). I also read about bent/broken valves. The engine has only 30K on a rebuild (9 years ago). This is the first time anything like this has happened and it came on suddenly about the time that the stick lifters usually quiet down.  Any suggestions?  What should I check?  Does it sound like I should put it in the garage for the winter and start saving for another rebuild? 

Thanks,
Scott