Cam follower? The big roller, or the slider? A bore camera down the push rod hole might be helpful.

Not standard fitment in the UK! Interesting. Thanks for sharing.

Probably not, unless there's something odd in the cam follower or push rod area.

Thanks for the updates....

Swiss? Or late emissions control engine. Most have a simple breather to atmosphere. Got any photos, curious!

Which engine is this for?

Rubber grommets on one of the bolts, allows for vibration mitigation.

Under neath. Suggest you invest in a high quality cavity wax and apply to the gap between the tank and the cradle, and apply liberally to all surfaces. I use Bilt hambler dynax S50. Other brands exist. NOT waxoyl. Buy a new fixing kit as well, cheap and cuts out the faff of recovering the old...

It's hard to tell from a video. Get a stick/rod mechanics stethoscope to isolate the knock. Sounds cam shaft speed, not crank shaft speed.

If it's in neutral, then can't be gearbox? Check valves first. I'd look at the dizzy, then the cam gear. Couldn't see from the video, is there oil coming through the rocker shaft?

Probably need a new tank. They are rust traps. But first. Don't trust anything about the restoration. Remove the seat. Remove the lid on the seat box. Take a good look around the tank. Especially when the bash plate cradles the inner tank at the front and at the back. Get underneath and...

I kept the series link rods. Well I welded two together. Just visible here, re routed to avoid a repositioned air filter.

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sounds okay.

The new chassis option is good, but an old chassis that's been blasted, repaired and galved is fine.

Get the chassis shot blasted. Any structural rust will be exposed. The rest doesn't matter, and the pre dip pickling will take care of it.

Totally. Current house was chosen exactly for that. Too old to be minding tools on the side of the road.

This is, for me, at any rate, the peculiar joy of a series. No matter how bad it is, no matter what the previous ten owners did, it can be fixed by me. And, if I do it right, the repair will last for longer than the life of most modern cars.

Time for a Toyota Yaris

Best? Electro rust removal. Cheapest? Knotted brush on an angle grinder.