Up and over garage door

[Greenmantle]

Double garage with Henderson S500N marathon up and over fibre glass door.
When open the door slides fine in the horizontal plane. At the tipping point it starts to judder and I have to give it a helping hand.
Obviously when closed I have to give it a helping hand to lift to the horizontal.

Can't see any issues with the chain.
It feels tight but I don't know how tight it should be.
I don't want to torque it any further in case I over tighten it.
I have partially stripped the carrier but cannot see the sprockets unless I totally dismantle the carrier from the chain rail.
There are no sprocket filings. I don't know if the sprockets are metal or plastic.

I am planning a strip down but don't want to be caught short with spare parts.
Does anyone know what I need to get and where from?
Thanks

[M138]

Tbh if it was me I would see if there’s a s/h roller shutter door for sale on eBay. Even if you found one slightly narrower you could screw a piece of wood to the brickwork either side like I did. Messing around with them old up and over doors seem a waste of time and money to me.

[Simpo Two]

There are repairers for such things. I once had an up-and-over door come off its tracks; a bloke came in and sorted it all out very easily.