Here are the best cost/weight mods:
- LiFePo4 battery - even if you take a big one to properly crank the bike even in cold temps it it will still save lots of weight
- Single disc conversion - sell the unused parts and you can make even some money!
- Remove crash bars and fit carbon/kevlar tank protectors instead
- 2-into-1 exhaust with titanium muffler
- 1.6"x21" front wheel with A60 rim and 2.5"x18" rear wheel with DID Dirt Star rim from the SE (slightly stronger than stock Excel IMO), both fitted with Tubliss inserts (these are made for 2.15" rims but fit ok with a thick sidewall tire)
The above all make the bike better handling and more reliable for off road riding (battery is arguable, although the Anti Gravities have a very good track record).
Going beyond this is going to require some really fancy materials and PowerCell like fabrication skills, or big $$, or a trade-off in strength or functionality (removing stuff or replacing with lighter).