Open IMGTOOL then click at "files" and then "Open"
and locate rockstar games\GTA San Andreas\Models\GTA3.img
And Click At "EDIT" And Then "FIND"
then a search will popup
type mtbike.dff and then search
now right click at mtbike.dff and then click at "Replace"
and locate this mod's mtbike.dff
then also do this with mtbike.txd hope i helped