I think you Use N-rage controller plugin in settings (ctrl+T). Configure the controller plugin through options menu. On the Devices tab, there's settings for various mouse sensitivities for movements in various directions (e.g. a setting for walk up, medium up, and run up).
I say "I think" b/c i remember trying to set up mouse for analog movement but never got it to successfully to work. I ended up getting an Adaptoid and real N64 controller and have NOT regretted it since.
This shouldn't be too expensive. An Adaptoid can be had as cheap as $10 or as expensive as $70. Average should be $35. I paid $25 on Ebay. I got a used N64 controller at EB Games for $12 and a Rumble Pack for $1. It costed me under $40, but meh, that's how much u'd pay for one N64 game. So it ain't too bad considering unlike reoccurring fees like games, controller related stuff is just a flat fee