First question is "how much"? We'll need some of the more experienced folks to chime in here but I don't know if you can replace this from the outside of if you'll need to pull and disassemble the gearbox. If the box needs to come apart you may want to do the R&R (mechanic speak for "remove and replace) yourself and take it to a GOOD transmission shop for the fix.

But if the transmission is not down on fluid very much and it's not gushing fluid you may just want to check it frequently and let it go for now.

Dan