Hi replaced the soft top last week went from black to blue .Looked at a few y tube videos that tended to go on a bit and missed a few pertinent points . The original seal seams to be a butyl mastic as you say and although mine was very sticky, it still pull away quite cleanly like blue tack as suggested.
You could try to gently heat it up with a hair drier or cut or scrape away with a putty knife or similar tool.
I used a panel trim removal tool to pry the hood hop from the rear plastic trim seal carrier.
The old sealer seems to have been a tape, like a dry glazing tape ,how ever i managed to find SIKA BLACKSEAL BT in a cartridge type tube and this seems to have very similar quality's being bulty based product and cost me $10 aus about 6uk .
The reason i say the old was a tape it had a regular size and even thickness and in some of the y tube videos used a 3m tape.And in compression seems to maintain a even thickness.
IF you think the product i suggested is for you please be very careful in its use ensure you Masking tape any areas you do not wish to get any on like the canvas its self.although can be removed from paint work with turps or white spirit but not the top.
I warped my car in cling wrap after cleaning so not to mark the rear of the car ,cheap and effective.
When using the blackseal cut and flattened the nozzle so to get a tape like shape of the blackseal from the tube on application.
Practice on a bit of cardboard.
Once the bead was on the hood i left it well alone over night to reduce the tackiness of the bead of product left on the surface .
The points i would suggest you pay attention to are.
Not all the screws are the same length.
Feed the hood and rail into position from the back towards the front being the doors and push the rear down into position best with 2 people
The back plastic rail has spacer washers on it that can move, and become a real pain on reassembly .
Place all fixing back in the right holes but do not fully tighten until you are sure the edges of the top have the right alignment and gap in relation to the doors,you will know when you look at the door edge!
You can do this by using sir- clip pliers to move the hood frame backwards or forward using the bolt as a fixed lever point ,the hole in the rail is elongated and if you stick the pliers against the bolt and hole edge you can move back or forward and saves a lot of frustration.Once in place tighten the front hood bolts up first doing one side at a time and the work back doing one left one right or visa versa until you you have finished the bolt up.
When i replaced the hood i used 316 stainless steel staples in the hood rail .however would think if pushed you could use just steel ones because the sealing bead should cover the line of staples and most likely not to corrode if sealed correctly.
The bead of blackseal i tried to maintain a even bead of about 6<8mm thickness and 12<15mm wide and on compression will spread and fill the gaps hopefully.
The rear plastic bit acts as a gutter and directs the water to the drain holes near the door pillar if you broke this on removal i would recommend a replacement one because once broken would not function correctly and drain all the water away fully
Hope this helps and wish you well with it .Cheers.
Mick