The chassis is not always ground.
It could be Q73.
CR4 - Thread: BST-80 FET NXP replacement?
http://www.supertex.com/pdf/datasheets/TN2510.pdf
Mouser Electronics
DIGI-KEY
Are a couple of places, but there are many others that will sell you these types of parts.
If you scrounge enough electronics from the dumpers you will find something with a mosfet that will do the job, but you could aslo order the correct part from someplace...... should be very inexpensive..... S&H will cost more than the part.
Maybe someone on the forum might do you a favor and put one in an envelope and mail it to you. (just a thought)
I would if I had one handy.
It looks to me like you could stack one on top of the other and increase the current handling by decreasing Rds (on)
Remember, if you don't have the skill for this type of work, it could be very easy to damage the new part or the pcb.(or both)
If you attempt this and make it all the way through, then inspect the pcb board after you are done to make sure that there is no errant solder any where else on the board that might have fallen from the tip of the soldering iron.(could short something out and cause it to not function correctly or damage other components when the radio is then powered up)