I assumed you had all the old tools already. Did you get what you need?
If you don't have to rebuild it with C18 you could always try XC8 in free mode.
It's slightly better than it used to be, but free mode still doesn't completely optimize things so that leads to folks saying "they purposely cripple it".
They don't... but without the latter passes thru the optimizer the first run passes can produce some pretty inefficient code.
It'll take some converting between the two versions to get it to compile (which is why I suggested just using C18 again).
XC8 can be installed to run with mplab 8.92, you just need to manually register one of the dll's.
To do that, open a cmd.exe window as admin and run regsrv32
Code:
cd "C:\Program Files (x86)\Microchip\xc8\v1.34\bin"
regsvr32 mplabxc8.dll
Modify as appropriate for your version