These are renowned for overheating and head gaskets can be a problem. If the van is smoking then it may well be the head gasket. Check the oil for water normally obvious by salad cream like sludge on the filler cap and a loss of water. Problem with these is that there is generally a bit of sludge on the oil filler due to condensation forming on the steel tube between the rocker and the cap, so you are looking for excessive sludge. Power loss should also be obvious. A compression test should also show a drop.

Overheating other than the head occurs due to many things but on these airlocks in the system is a regular occurence. It took me some time to get mine under control and a scalded arm from removing the pressure cap under the seat when the heat was up. The steam hit the roof of the van via my hand but it cleared the air lock for me and it is now ok. Incidently mine was not smoking so I| did not suspect the head gasket. Hope this helps