I agree with what the other guys said, but in addition, I'd probably let it break itself in without any load on it first... Did the guy who rebuilt it do any breaking in? Or just put it together, and then give it back to you? Since it's not a COMPLETE rebuild, it might not need to be broken in as much as something that has new rods and what not, but still. I'd change the oil sooner rather than later, especially if it hasn't been changed since being put together(maybe 250-500miles).

And on the initial break in, I think it'd be best to let the engine sit just above idle speed in your driveway or somewhere (make it hold steady rpm somehow) for about 45min-hour to let everything start to get used to working together. Watching temperatures the whole time, to make sure nothing's going wrong(as far as you can tell with only a temp guage that is). Then, let the engine completely cool down, and do another session of the same sort, but at a higher rpm. Possibly starting out at a lower rpm, then picking it up some more later on in the current session.

Anywho, I'm basing this off of some stuff out of this thread:
http://forums.turbobricks.com/showthread.php?t=62357&highlight=building+b23FT

Start reading posts(mainly TheAspirator's and StealthFTi's posts...) around post 185. VERY good information in there if you ask me.