[Chas]

The dried hardened wire insulation can be softened with a heat gun, not SCORCHED, then the wire will be pliable without flaking the insulation while hot. It will become brittle again when cooled.
Since you have the transformer off the chassis the asphalt potting can be melted off in a lab oven set for around 375F. Yes, the asphalt will burn with high temperature heat gun.

The best way is to secure the case with the opening down, place a bread loaf foil tray under the case and allow enough gap for the transformer to slide out. Back up the arrangement on a lower shelf with a large foil roasting tray..

An alternate is to dissolve off the asphalt with odorless mineral spirits. That can take weeks..
The heat gun can be used to extract the transformer by suspending with the wires and warming the can with the heat gun. Once expose CAREFUL playing of the heat gun will get most of the asphalt off then the rest can be dissolved off with the mineral spirits...

Be careful how the melt out is performed, wear cotton gloves that can be discarded.

Keep mineral spirits away from the oven and the heat gun. Spirit vapor will decompose in the hat gun or oven and make nasty toxic fumes and too much fumes are explosive.

FWIW There is one Antique Radio Forum member who re-pots with asphalt. Has no issues doing so.

https://antiqueradios.com/forums/memberl...file&u=514