Can i put a diesel engine in my 87 vw cabriolet?

All Volkswagen inline-four watercooled engines have the same mount points, so the diesel you want will definitely fit. It will work so long as you take the transmission from the donor car too. If you use a gas-engine transmission with a diesel engine, the car will be REAL slow. And then there's the other problem, namely that diesel is about $1 per gallon more expensive and VW diesels don't get enough better economy to compensate for that.