I have a garage door with a wooden frame. It's about 30 years old and
the bottom 6" of the frame have rotted at either side.

Is it's quite a major task to replace the frame I wish to do a swift
repair, something that'll last for another 5 or 10 years and look
cosmetically acceptable.

I thought about removing the bottom portion with a handsaw and fitting
a new piece of timber. Unfortunately, there is a metal strip screwed
to the back that also supports the door!

Has anyone got any clever ideas for cutting the wood without removing
the metal?

I'd thought of simply screwing an 'L' profile metal/plastic extrusion
over the top of the rotted portion. Any gotchas with taking this
approach?

s