Let's offroad / mountain bike / mtb / ride dirt

Hive mind opinion needed. I’ve got a rigid boost Fuji I built from bits cheap that 95% of the time is fine for what I ride- bit slow on flats and draggy climbs with the big tyres, fun on easy downhills.
Stupidly I’ve entered the peaks Scott marathon in September so I’m now thinking my bike will slow me down! I’m now torn between

1. Get 29er boost wheels for it and thinner tyres - similar height total, faster rolling but less cushioning
   
2. Get a sus fork for it
   
3. Get a new rear mech, cassette and shifter so I can spin up hills more- currently got old 9 speed bits
   
4. Stop wasting money and somehow borrow or buy a proper xc marathon bike for the day?
   
   

What we think?