Peter Drew

Yes, you can use a 50mm aperture refractor or larger. The larger refractor will probably be a better telescope but it will still operate only as a 50mm. 🙂

Are you sure that it is the blocking filter that needs replacing? It's usually the ITF filter, the one that is fitted before the blocking filter that fails and needs replacing. 🙂

Could cable management have anything to do with the problem? 🙂

Referring back to the original photo there is probably another clutch between the motor and the worm shaft, this is to protect the motor should something restrict the rotation of the wormwheel. It is the bushing next to the motor output. There are two rows of grub screws forming the connection, one set possibly fixed to the motor shaft and the other connecting the worm shaft with some facility for slippage in the event of a problem. If this is the case it is another area for unwanted drive variation. 🙂

It probably needs at least some grease as the clutch has to be able to slip in the event of something impeding the rotation. The "trick" is to adjust the slippage torque to avoid slippage in normal use but also to avoid locking up. Try tightening the nut a small amount. 🙂

If the brass worm is turning then the wormwheel must also be turning which points to clutch slip as already diagnosed. 🙂

Give it a good sanding down and then a coat of silver-grey Hammerite. Available as a spray can 🙂

I'm afraid this is indicative of a de-contacted etalon, the etalon plates are held together by molecular fusion and cannot easily be re-contacted other than by the manufacturer. ☹️

I was given one of these to "play" with some years ago, it is a "Bird-Jones" type. It actually looked quite well made and finished but the performance was pretty sub par. After faffing about with the collimation for ages I managed to get a fairly decent image out of it, good enough to please a complete beginner but not worth spending any extra money on it. Quite a bargain for free. 🙂

I fitted an AM5 to the top of a "Todmorden" pier by spacing with a ring cut from a 5" diameter 1/4" wall thickness tube between the bottom of the mount and a plate on top of the pier. The 3/8 screwed rod pulling it all together would take some shifting to accidentally rotate the 5" ring. 😀

Learning to collimate at F4 will be the least of your AP problems. 🙂

Yes, that will teach us to not scoff at Astrology. 🙃

Google Mel Bartels for further insight. 🙂

To make any headway with this proposition you would have to consider an ultra-lightweight primary mirror. 🙂

Very difficult to assess a repair potential without having the tripod to hand. High grade aluminium doesn't usually snap like that so it's probably the usual "pot" metal casting.