First thing you need to do is create backup for this following files :
- sanchez.dff & sanchez.txd, both files are inside gta3.img, extract both files using spark or IMGTools
- hadling.cfg & carcols.dat, both files are inside "<GTS SA Root Folder>/data"

To install this mod, you can use tools like Spark, IMGTool, and etc. The tutorial is available on Youtube, you can find it yourself.

To use this mod, you must modify the handling, by following this step :
1. Open "<GTA SA Root Folder>/data/handling.cfg" using Notepad.
2. Find "DIRTBIKE" by pressing Ctrl + F.
3. Replace all the line with following line :

DIRTBIKE      500.0     195.0   5.0    0.0 0.05 -0.09 103 1.6 0.9  0.48 	5 190.0 50.0 5.0  R P 	14.0  0.50 0 35.0  	0.40 0.15  0.0   0.05 -0.06 0.5  0.0		0.0  0.15 10000 	1000000		0			1  1	7

4. Save & close !

Also, I set default vehicle colour in 5 colours variant (Black, White, Red, Blue, and Yellow), follow this step to apply :
1. Open "<GTS SA Root Folder>/data/carcols.dat" using notepad.
2. Find "sanchez" by pressing Ctrl + F.
3. Replace all the line with following line :

sanchez, 6,6, 0,0, 53,53, 3,3, 1,1

4. Save & close!

Enjoy and play!

Mod stickers can also be replaced manually.
If you want to change the sticker on the body (I have included the template):
1. Create right and left sticker designs according to the template in the .BMP format, also make the alpha mask file in the .BMP format (Examples of alpha mask files can be seen at sanchez.txd).
2. Open sanchez.txd using TXDWorkshop, look for textures with the name "r_stripe" for the right sticker, and "l_stripe" for the left sticker.
3. Import each of the following stickers with their alpha mask.
4. Save and close TXDWorkshop. Then enter the "sanchez.txd" file that you just changed the sticker into gta3.img.

To change the frame colour, just change the "base_paint_frame" texture inside "sanchez.txd" file.