No way throwaway Roger! Riveted, yup - so are Lucas - but correct rivets can be had.
If you don't want the BTH item, I'm sure you could get some useful £$€ for it, even if it's tired, as they don't grow on trees. Triumph owners particularly look for them, as a lot of the '50s pre-units had BTH KC2 magnetos. I've seen some wear on them, sure - flats on the brass rollers and so on, springs with a busted hooky end as I mentioned (and we know how trying to form new hooky ends on clock springs goes - badly, almost always!) but there's a demand for them and there's a steady market for new fibre pinions for them.
On the binding front, depending on exactly 'how tight' etc, could be it's not actually abnormal - CheeserBeezer's Priory Mags website has the best summary of some of the issues, under'A Bit of ATD Theory'. (Of course, if it jams solid, it's definitely not normal though!)
You could replace it with the Lucas version, yes, as the tapers and threads are the same. I'm pretty sure pinion alignment would remain good but I don't know what the degrees of movement are on BTHs, or the choices that were available. But I do know that the Triumphs had 15-18° on their Lucas-equipped versions, which is more than Beesas' 11-13°. That's at the mag, so double it for the crank.
