G'day.
I've done quite a lot of master creation, moldmaking, and subsequent production run resin casting, so I just might have an answer to any question you might have.
Scratchbuilding the replacement details can be easy or hard - just depends on the subject and level of detail. I often use resin, plastic, CA glue, epoxy, aluminium and steel to create the masters. As the rubber molds degrade after a certain number of resin castings, I find it better to have made the master from sturdy materials that can withstand several mold creation processes.
Also, the type (or shape) of master will determine the type of mold you need to make. Complex shapes will require a different approach - masses of fine detail will require a much softer RTV rubber for the mold.
In truth, I ended up getting all my skills and knowledge from 3 places - FSM articles, the staff at the store where I purchase my resins and rubbers, and experimentation.
Now, the materials are really expensive, so it's not a good idea throwing caution to the wind and jumping in with both feet and both eyes closed.
Ask questions about what's the best resin to use for modelling, and what the best RTV rubber is for the requirement from the guys at the shop that sells the stuff. They talk all the time to the people who use it, so they might be able to direct you to products or processes you might not have thought about. Use their knowledge to your advantage.
Also, do a search (click the word "search" at the top of the page) for resin casting - there's an article in the Nov 2002 FSM about it.
Also, have a look at Alumilite's website:
http://www.alumilite.com/
Look at the top of the page and click on "how to". There's a series of pages that show how to do lots of different stuff.
If you would like to go into this in more detail, please email me on the link below my post - I'll be happy to give you any help I can.