I recently blew up my 2021 Gas Gas 300EC racing Last Dog Standing Extreme Enduro Race in a silty dusty hill climb (most likely a filter problem) see video here. I had to get the Cylinder replaced and redo the bottom end in 3.5 weeks before the next race in Idaho: Silver Mountain Extreme. Below are all the links and resources I put together while tackling the rebuild for the first time. It was all manageable just takes some time and a few special tools. I filmed the whole rebuild process as a how to but not sure when or if I'll find time to edit and post it on the Trailboundco YouTube 

Disassembly Videos
Tokyo 2017 300
RMATVMC 2015 300 
RMATVMC starter motor 2017+
RMATV bearings

Assembly Videos
Tokyo 2017 300
RMATV bearings 

RMATVMC 2015 300 part 1
RMATVMC 2015 300 part 2 

 Parts
(Might add a more specific parts list later)
RMATVMC Bottom end rebuild parts

Tools

Disassembly Tools

• Gear puller that opens to around 3.25 wide
• Special pliers for removing starter motor seal ex: this video 30:40 , normal pliers did not work but you can get away with hitting the starter motor out but feels a little more sketchy. Later found this RMATVMC video at 9:00 showing using 2 90* picks to get it out
• Small punch for moon lock thing
• Gear jammer: But if you havean old motor gear you can cut a pie piece out of it, thats what I used but wished I had the tool 
• Clutch holding tool, this is not necessary as you can use a gear jammer on the other side
• Flywheel puller 26mm x 1
• Crank case splitting tool 
• Seal puller 
• Huge breaker bar/cheater extension or 18v impact gun or pnematic impact gun 
• Snap ring pliers for cir clips if you remove bearings 
• 24mm socket 
• Gasket scrapper + parts cleaner: razor blade works or fancy version: KTC ceramic 12mm KZ4-12
• Bearing cleaning: kerosene + brush
• Powervalve cleaning: Chem Dip Parts cleaner 

Assembly Tools

• Tusk bearing/seal driver or socket, or use old bearings (1 1/4” for one small side of crankshaft), driver set would hav been nice
• Theres a blind flange bearing that you need a special tusk tool and slide hammer for but I dont think I replied it
• Propane Torch 
• Thermaturer gun to heat to 100* for bearing install or just spit on it until it needs up instantly, should be able to touch it quick 
• Waterproof grease 
• Assembly lube 
• Loctite  
• Might need a friend to help run through the gears but I didnt disassemble my gear box, left it zip tied together
• Troque wrench for 4.4, 7.4 ft/lb 44.3, 73.8, 88.5, 110.6 
• Small torx socket (so you can use torque wrench)

Tips for next time:
If you dont have a strong impact driver (Milwaukee will work) leave the motor in the frame and loosen the really hard bolts first before removing the motor. 

Cylinder Plating 
It cost's round $200-$300

Millennium 4.5 weeks  (this is what I used, more details below)
Powerseal: 5-6 weeks
LA sleeve: 6 weeks (I think they said they dont have a sleeve for the 2021 yet)
US chrome: 4-6 weeks but only working with dealers right now
Q and E in Anaheim, CA:  2-3 weeks 

Millenium Plating:
2 stroke plating: 3-5 weeks $255
Rush order: 9-12 days +$75: $330
Double rush order: 6-9 days +$150: $405
-they can order a piston (to match it). But they won't order it until your cylinder arrives so I overnighted them cylinder to get the project going ASAP. 

New Cylinder
Most likely a new cylinder is sold out but try these places.. 

RMATV OEM cylinder + piston $1037.46
Slavens Full kit cylinder + piston + powervavle + gaskets + SX high comp head $1000 
Judd racing cylinder + piston $1005.40 shipping is 3 days from uk

Other places to check 
AMOC
Dennis Kirk 
Motosport
Call random shops they might have a cylinder in stock by chance

Crankshaft Install People 
It'll cost around $150 

Ask  around locally you can probably find a metal guy with a lathe and the balancer needed to do the job. Your local moto shop might know a guy they most likely outsource it themselves. 

-When you call the metal shop ask if they do crankshaft bearing install and balancing 
Tell them: 
-I’ve have the crankshaft assembly out of the bottom end
-I have new bearings and a new crankshaft 
-I need the bearings to be replaced, pressed and balanced 

Parts (to bring to the metal guy with your bottom end)
Rod kit $72
Crankshaft main bearing $71
Or oem: Crankshaft: #288 #54830015244

Millenium crank shaft: 20 days $101?
Probably super expense to ship both ways because the bottom end is super heavy!