Replace/repair rotten doorstep

[Jilly]

Is there an easy way to do this? It’s an old door which needs replacing, but I can’t do atm. 
The  bit on the right is rotten but the rest is ok. 

[Redbeard]

Yes, ish...

 

I'd usually get my local wood-wrangler to make me an L-shaped section comprising say 200-300 hardwood for the cill and 150-200 for the presumed soft bottom of the stile. He would do the fancy joint, and I would do my best not to screw up 2 diagonal joints to join new to exg.

 

Or, less 'subtle', and assuming that the 'stop' part of the cill is not badly rotten, just saw/chisel it out square and screw/fill a piece in (bevelling as required for rain run-off), and ditto (less the bevelling!) to as much of the stile as req'd.