I planned a similar trip a few years back but got sidetracked at the last minute. View my plan with a healthy dose of skepticism as I never actually made it….
It was my understanding that Codemasters purchased the old Sierra building after the 99 shutdown. I searched for Codemasters (in some manner) on Google maps at the time and identified what I thought was the building. I just tried to do it again for you and end up looking at the side of a mountain between three trees somewhere east of Coarsegold. Somehow that seemed wildy appropriate.
My advice? Shoot Ken Williams or Al Lowe (email@example.com) an email and ask for directions or at least some landmarks. I’m sure they would be willing to help you out.