approach for creating an original region based hack

[lotta65]

I will be using Fire Red as a base to construct an original region for my hack, but I have a question about the best way to approach it. Does it make more sense to use JPAN's cleaning patch and write 100% original scripts from the ground up (I have a limited understanding of what everything in the games scripts do when I reference them, so I'm concerned if I start from scratch I may miss something important, or use the wrong flags, etc) or does it make more sense to leave the games scripts intact and try to rebuild them with my own routines.

Some of the main ones I dont understand are most of the map scripts, and why they exist. (Some are very self explanitory, such as oak checking your pokedex when you enter pallet town, but others such as this one baffle me. This is "on entering map/not on menu close script #0 for pallet town."

Spoiler:

'---------------
#org 0x165465
setworldmapflag 0x890
checkflag 0x291
if 0x1 call 0x8165488
compare 0x4070 0x0
if 0x1 call 0x81654A8
compare 0x4070 0x1
if 0x1 call 0x81654A2
end

'---------------
#org 0x165488
checkflag 0x83E
if 0x0 goto 0x81A77A9
compare 0x4070 0x1
if 0x4 goto 0x81A77A9
setvar 0x4070 0x1
return

'---------------
#org 0x1654A8
checkflag 0x291
if 0x1 goto 0x81654BD
movesprite2 0x1 0x5 0xF
spritebehave 0x1 0x7
return

'---------------
#org 0x1654A2
setvar 0x4070 0x2
return

'---------------
#org 0x1A77A9
return

'---------------
#org 0x1654BD
movesprite2 0x1 0xC 0x2
spritebehave 0x1 0x8
setvar 0x4002 0x1
return

I feel very lost navigating the games scripts most of the time, maybe there is a tutorial someone could steer me towards to help with that?

Anyways, I'm rambling. My apolagies. Any help with this would be much appreciated. Or tips in general. :)