I play on a minecraft 1.6.4 server a lot (tekkit main) however the only way to sprint is by double tapping W.
This mod will simply allow you to start sprinting when you click shift (holding shift will keep you sprinting too)
Currently, i haven't added an option to change the key... you will just have to remove any key bound to shift. e.g. if you use shift for sneaking, make it CTRL (or if you have a mouse with buttons on the side, aka the navigate buttons, use one of those for sneaking)
like any mod... simply drop the file into your mods folder and it should work
The mod isn't laggy as well, its pretty much less than 100 lines of java code. all it does is check (every player tick) if the shift key is pressed, and if so, set the player to sprinting. quite simple :)
Also, to compile the mod i used forge 1.6.4-9.11.1.953 and mcp-8.11 (aka MC versions 1.6.4). it tested it on the newer forge version (1.6.4-9.11.1.1123 i think) and it worked. I haven't tested older MC version (e.g. 1.2.5) but it should work considering forge is quite backwards compatible, and how small the mod is
todo:
mods like the powersuit mod (MMMP) do something really odd with the game which breaks the FOV change. i dunno have to change the FOV of the client so thats something that i cant fix :(
