So, I have spent more hours then I should have on this problem. As always problems like this always turn out far more complicated than you hope.
So this is the essence of your speakers.
The bass section is a traditional reverse tapered transmission line. This makes it a rare animal. However it gives a bass accuracy quite unlike any other form of loading. For the size of the speaker bass extension is excellent.
F3 is 30 Hz. A TL properly designed rolls off only 12 db per octave, so this speaker will have usable good clean bass to 20 Hz.
Another unusual feature is that the mid range driver is also in a TL that is totally damped. I always thought I was the only one to have done that, now I find apparently not.
Now the speakers come from an era where a lot of people where using and experimenting with "simple" first order crossovers. This means that the drivers cut off very gradually only 6 db per octave. So for instance the tweeter crossover at 5 KHz is only down by another 6 db at 2,500 Hz. The object behind this was to preserve time and therefore phase coherence to try and create the so called time aligned and therefore minimal phase shift speakers. They were also referred to as transient perfect speakers.
At the time of your speakers and a bit before this had quite a following no only from your speaker's designer, Pat McGinty, but the likes of JIm Thiel especially and Dynaudio in Denmark. There were also others. I had my flirt with this also.
The problem is there are significant downsides. This part is important as it relates to your problem. The issue is a lot of driver overlap either side of the crossover and as a consequence a lot of power to the driver either side of the pass band. This is an especially serious problem for tweeters.
Now to your speaker the original BH speakers.
The tweeters are the Audax gas filled piezo tweeters. After lots of searching I have managed to find the original
Audax spec. sheet.
Now if you look at the graphs you will note that the unit is really a super tweeter and only usable above 5 KHz.
You note apart from being a superb unit, that it naturally rolls off at your speakers 5KHz crossover point. In addition piezo tweeters tolerate a lot of out of band power. So it is easy to understand why this unit was selected.
The other thing shown on the spec sheet is the driving transformer and circuit. This will have to be removed if you plan on having these tweeters repaired and modified. I suspect they all fail as the designers forgot that gases slowly diffuse though thin membranes.
Now your speakers used Audax 3.5 mids with aerogel cones. Back in the day those drivers were actually very good. Unfortunately I can not find the spec sheet on that particular driver.
Now as to your second option, just changing the tweeter does not make it a BS2. The BS2 also uses a
Scanspeak mid range. So to make it a BS2 that unit would also have to be changed but it is much bigger, so probably not possible as it has to fit in the small TL. Also since I can't find the specs on the Audax mid, I can't begin to guess at the crossover mods.
So that leaves the only option other than original tweeter repair to be replacing the tweeter.
Now the recommended replacement is this
Scanspeak tweeter. The biggest issue with this has been tweeter burn out. This is because of the large amount of power sent to the tweeter below crossover because of the first order 6 db crossover. However the acoustic response of this tweeter makes it suitable.
Now I have found these speakers have a had a significant following in the UK. This is not surprising because of a fondness for transmission line speakers. So a UK site has recommended this Scanspeak tweeter as replacement. However I feel it does not roll off nearly fast enough at 5 KHz to make it a good replacement. I think it would adversely affect the sound significantly. However after removing the transformer and the high pass section of the crossover putting a 5.1 mfd cap in series with the tweeter would be all that is required according to my modelling.
I think the better choice though is the ring radiator tweeter and just not play the speakers too loud.
If you choose to use that tweeter then I calculate that after removing the transformer and high pass filter, then you need an 8.2 mfd cap plus a 1.5 ohm resistor in series with the tweeter, you need a 15 ohm resistor between the +ve and -ve terminals of the tweeter.
I actually think this would most likely work quite well as it is close to the response of the original.
If you want the speakers to sound exactly original then you need to contact MOCA France.
I expect you are going to have lots of questions after this, so I wait in trepidation. I have given you accurate information, as usual the web is awash with less than accurate information.