Here are two mods of M4/Ak-47. Actually these mods are for M4 but you can still use it for AK-47.
http://www.thegtaplace.com/downloads/f5127-colt-m4a1-carbine
http://www.sendspace.com/file/8iwpq7
Or just google it on google but if you type gta san andreas m4 mod you will get more mods.
Just rename the m4.txd and m4.dff files to ak-47.txd and ak-47.dff. And just replace .txd and .dff files with original files by the help of IMG tool or with GGMM.