The Lambretta chain guide has been made many times, by many people and in various forms. The ability to have the chain guide pull the chain down as well as push it up has given us the chance to dump the worn chains for new ones.
One of the recent versions from Casa in Italy has a nifty feature. It comes with a base that bolts down to the casing and can be left alone while the top piece can be moved around to either push the chain up or pull the chain down, leaving you to use any sprocket combination you want. It also bolts down over the chain and not under so you don’t have to remove the chain or sprockets to take the guide off.
I’ve been using one for a year and thousands of miles with no issues to report at all. It’s solid and strong. It’s stayed in place with no movement at all. I have been using Loctite on the screws as suggested by Casa for added security, but then I do use various strengths of Loctite on most of my Lambretta screws and fixings.
You can get the Casa X44 from most Lambretta spares stockists.