The fact it happened when you swapped motors says to me burning oil/water. More then likely oil.
Keep an eye on your fluid levels and all should be good soon, id say a good oil flush would help but anythings possible. perhaps try the next grade oil higher 15/45 or 10/50 and see if that helps. Keep an eye on the plugs aswell, build up of soot or wet plugs can tell you all sorts of things about an engines health.
Could be moisture build up in carbon in the exhaust, that tends to happen with mine i only drive it 2-3 times a fortnight, blows alittle whitish then stops.
As with any Toyota engine eventually the valve seals get harder with age thus loosing their sealing abilities.