Welcome. The Rotoscythe engine may not be a Villiers, I know early Rotoscythe machines used a Bux engine.
If you could post a photo of your Monrotiller and if it has it the machine number I can date it and give you some more information. Monrotillers are my speciality. It might be worth starting a new forum post for the Monrotiller.

Hi there, what a find!
It looks like you have the fore runner of the Rotoscythe Eton model, you are correct to say villiers engine on that machine as yours dated from roughly 1935,Although its lost it original colour on its hood.Rotoscythe did use Bux engines from 1939 onwards. Hope this helps

A repair like that is all part of the machines history and demonstrates the old ideal of mend and repair, unlike the throw away society we now live in.

Good advice about the nuts in fuel tanks , I cable tie mine into a cement mixer , run it for half an hour and it’s job done.
Ignition parts will be easy to obtain , they are still all readily available , Meetens are my personal choice.

Andy.

Have you ever used vinegar or the off the shelf methods?

I usually use electrolysis. It removes loose rust and gunge and neutralises anything remaining in nooks and crannies. It’s also non destructive to soldered joints etc.