Sorry about your blown heater. I've converted 4 Empire engines and one
Weeden to cartridge heaters. I you could start looking thru the ebay
listings for cartridge heaters and find one (or two) that will deliver
at least 300 watts. I suggest you finagle together two pieces of flat
steel or a thick rod, drill a hole equal to the length of the cartridge,
put the cartridge in, and then fit/wire it into the underside of the boiler.
The pieces of steel act as a heat-sink/moderator keeping the cartridges
from going incandescent and burning out. This latter instruction is used
when you are replacing a blanket or 'hotplate' heater.
If the engine has a cartridge heater (at least one Weeden does and I replaced it),
try to find a close to exact replacement and you will not need the additional
iron.
Good Luck,
Wayne