That manifold needs some serious cleaning- - - - -either a dip in a commercial hot vat, or some time in a machine shop's power washer, etc. All that crud will eventually end up in that new engine, with some really bad consequences. I do my own "hot tank" cleaning with a strong solution of water and lye in a 55 gallon oil drum, with a wood fire built around it. Greasy residue gets a trip through a similar hot bath, with Trisodium Phosphate as the cleaning agent.

Propane isn't hot enough to free up a stuck heat riser- - - -use acetylene. Heat- - - - -tap with a hammer- - - -"rinse and repeat"!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!