Assuming that coh1 replays work like coh2 replays (and Seb´s description sound like it does):
Every computer RNG is actually only a pseudo-RNG. You have to "seed" the generator with a number. Seeded with the same number, you´ll get the same sequence of "random" number. The replay files do contain that seed, so when replaying the replay, each event that requires some RNG role will give the same result as when the game was played originally. Like, the units will hit, miss and penetrate like they did originally.
After some header, the replay basically contains only the commands of the players. So, the replay file ends when the players stop issuing commands. This also means that the file does not even contain explicitly why a game ended and who won. That is, unless one of the players issued the "surrender" command, which is stored in the replay as well.
I see. However, what is exactly is the start header? Are the ticks variable depending on the replay or are they the same, but can be masked by player commands in the replay?