I usually rebuild them, but I have done just a replacement before with no problems. As long as the main cap isn't deformed and still fits tight in the block when the bolts are removed with no obvious signs of damage, you should be ok. We bought an LML a couple of months ago that had a slight knock in the bottom end; after doing some inspection and teardown on the motor, we found signs that it had broken a crank and had damaged the main cap causing excessive main clearance and the knock we were hearing.