If I were hell bent on spending the money,
1. Downing Atlanta Supercharger, still have some change for maintenance.
2. or a Metric Mechanic engine, haven't looked at their site in a while but they
used to have some drool inducing stuff in that price range.
If I were really in a position where I hadn't already done either of those things what I'd really do is take care of any maintenance/reliability issues I had, replace any worn rubber and put the rest into savings. That would be your elders talking though. Not as much fun but it is really kind of crazy to put a lot of money into what is probably a sub $3000 car.
lol hahah come on guys am serious what would you do if you had 6k to work with to mod your m42s peas!