The roamer in FR is released into the world by special 0x129, at x141D9C.
Its species is chosen from the bytes at x141caa, x141cb4, or x141cb8 (xF3, xF4, xF5 respectively) based on your starter choice (var x4013).
Its level is dependent on the bytes at x141cc8 and x141cdc. I advise changing both to the same value; if only the former is changed it gets generated as a level 50 mon with incorrect HP.
The ability to track the roamer in the dex is specific to Raikou, Entei, and Suicune. There is a table at x4642F4 that maps values of var x4013 to dex entries that should instead check the roamer. To make the dex acknowledge one roamer and disregard starter choice, change the first value in the table to your roamer's ID, and put four 00s at x13cb80.
No. By doing that you'll have just made it special 0x0, which is just a different special. I think that heals your Pokémon actually, but don't quote me on that. To remove the script, just delete the events with A-Map.If I want the event to never happen, do I open a hex editor and go to the offset at 141D9C and change the value 81 (which I'm assuming is flag 0x129) to 00?
No. By doing that you'll have just made it special 0x0, which is just a different special. I think that heals your Pokémon actually, but don't quote me on that. To remove the script, just delete the events with A-Map.
~EDIT~ After checking there, there's already a 00 there. So I'm not sure where you're getting the 81 from? And besides, I also realized after I posted that the 0x means it's already in hex, which means it would look like 29 01 anyway. And I'm fairly sure after reading his post that flags aren't really involved.
Now, if you want to remove the part that causes roaming specifically, but maintains all other functionality, you can go to 0x171035 and replace the 25 29 01 there with 00 00 00. This just replaces the special command there with three nop commands (which do nothing).
What do i change and with what i cant find that script Celio has script number $170F3D so do i need XSE editor or hex editor and what do i edit in script?