It is far easier to do this if you have access to both the original (Tasking) and the "target" compiler - that way you can check that any changes simply port the source without changing the actual operation.

Could you use the demo version of Tasking for this?

You're going to have to read the Manuals for both Tasking and your chosen target compiler (sdcc, keil, iar).

Then, as Erik suggests, you'll have to work through changing all the Tasking-specific extensions into the corresponding specifics of your chosen target compiler.

You'll also have to check for any other reliance on implementaion specifics - such as type sizes, byte ordering, etc.