Replace the sanchez.dff and sanchez.txd. You can use e.g. "IMG Tool" or "GTA SA modinstaller".

And replace the vehicles line and the handling line.
You can find them in your "data" folder. (default path (english OS): C:\Program Files\Rockstar Games\GTA San Andreas\data)
				         (default path (german OS): C:\Programme\Rockstar Games\GTA San Andreas\data) 

BTW: You can choose between a dirty one and a clean one ;-) 
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vehicles.ide

468,	sanchez,	sanchez,	bike,		DIRTBIKE,	SANCHEZ,	biked,	motorbike,	5,	0,	0,		23, 0.68, 0.62,		-1


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Handling.cfg

DIRTBIKE      400.0 170.0 5.0 0.0 0.05 -0.09 103 1.6 1.5 0.48 5 250.0 70.0 5.0 R P 20.0 0.50 0 30.0 0.85 0.15 0.0 0.15 -0.16 0.5 0.0 0.0 0.15 10000 1000000 0 1 1 7

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P.S: Please do not upload this bike to any other webpages without my permission. Thank You!
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Husqvarna TC 610 (1999) by  Sh!t H3@D

Nickname: Sh!t H3@D
E-Mail  : leerkopf@live.de