Before you order new parts, remove the red wire from the transistor and pull off the connector plug. If the transistor case still reads any resistance other than open in reference to the chassis, then you know that the insulator or the washers have failed.
The insulators are made from mica and are available from places like Mouser. They may also be sold on Amazon or eBay. Look for TO-3 insulator or TO-3 mounting hardware.
Please check the nylon step washers on the two mounting screws as well, because they can crack and cause the screws to touch the chassis as well.
If you need them and can't find them, I can look them up for you.