Clunking: The clunking COULD be the tailshaft bushing. Another possible cause is the driveshaft center carrier bearing and/or rubber cushion are shot.

Oil leak: Maybe they got the seal in backwards? Or pushed it in too far? Be sure to verify the crankcase ventilation is working properly. If the flametrap becomes clogged, the resulting crankcase pressure can blow the seals out. With the engine idling, you should be able to loosen the oil filler cap and fill a slight crankcase vacuum sucking it down. (Known as the "Jiggle Test" here on the Brickboard.)