This is the one of the biggest 510 forums that I've seen.....
http://www.dimequarterly.com
This is were I sold mine. Just go to the classifieds and post your KA. I beleive that the majority of the people on there are looking for a pretty much complete swap (i.e motor,transmission,ecu,wiring harness etc.......) but there people that are just selling the blocks and so forth. There are people on there selling SR's as well. Oh yeah, just to let you know.......if you do sell your motor to someone on that site, keep your motor mounts!!!!! They have no use for the KA ones so you might as well as keep them.
:ylsuper