1) another 2.6 & the 2.6 turbo from a starion/conquest would work.
2) don't know. check your owners manual if you can't get an answer here quickly enough. It may be your dipstick tube.
3) Reset the speedo? What do you mean by that? To resolve power issues or other things? For the power issues, you should check the condition of your carb and confirm there are no vacuum leaks as the stock Mikuni is highly affected when they exist. Also, don't start immediately tearing out the stock vacuum lines w/out properly documenting what you're doing. Otherwise, you're bound (highly likely due to it's complexity) to screw up putting the lines back. If you do replace the lines, do them one at a time. Don Huysman did a great job documenting the carburetor setup with diagrams. I no longer have the links to those (I don't have a 2.6 so I'm not focused on remembering them) but maybe someone else who has them can post them up or post up a link to them.