I've just had a quick look for replacement tweeters and couldn't see any MS1T-3R5 but saw some similar to that posted by stevie that have a very slightly different name - it's probable that, if they physically fit your cabinet, they'll be fine and you won't notice any difference.
Another option* is a new (to you) pair of speakers I have bought a few bits from Wilkinson's and they are a proper old-school hifi shop. It would also be worth giving them a call and asking as they may well have a spare tweeter (they have hundreds of spares and items that aren't on their website).
*If you do go for these and they won't post, I will be visiting them in a couple of weeks so would be happy to pick them up and we can sort out getting them from me in north Wales to you in the W Mids.