To do that first you need to
Method 1 -Using Manual Installer
SAMI(San Andreas Mods Installer
This tool will directly backup and replace the file dff and txd in the gta3.img
And change the handling. This tools can only install vechiles mods.
Example: I download this bike mod from gtaplace
This mods will replace FCR-900 as the ghost rider bike.Theres two type of selection in this bike.
1.with glowing tyre
2.without glowing tyre
Download, Install, and Run the San Andreas Mods Installer(SAMI)
Then choose your gta directories and browse your mods (rar files)
Make sure you check box Backup files
There was a two selection in this bike mods like i said
Chose which one will you use.
This tool can edit the handling itself
After that choose which bike will you replace. Remember bike must be replace with bike dont replace it with bicycle or car or it will crash the game. You can enter the custom name of your mod installed.
Its easy if you want to uninstall it.
Finally click install.