As I said, send an email to Amain , carolinas or other online Hobbie Shops and sk about mugen or ofna springs (mention the diameter and legth).
Also, the seals at the bottom of the shocks (where the shafts go in and out), are supposed to SEAL the air out and SEAL the oil in (that's where they got their name). If they don't do both things, they need replacing.
After that, refill your shoks (I'm sure 1 bottle will be more than enough) using the method I explained futher up, and you must notice a big difference. If not, you could always buy a set of shoks of a different brand and put them on this buggy....