for it to be aesthetically pleasing I would as said cut the nipple off & NPT tap it for a plug (I'm assuming you ain't never gonna use the heater again with this engine/pump) like in the pic & first stuff a piece of rag in there to catch the shavings. If the (final) appearance ain't objectionable as said the piece of heater hose with a plug/2 clamps would be the easiest/fastest.

live every 24 hour block of time like it's your last day on earth