as 99Y has already said, there are low-corrosivity fluxes out there; one in particular that I like is a 'tinning' flux (has some solder in it) which has very good cleaning and wicking action. The active ingredient is water soluable and can be completely removed with soap, water and a sponge or rag. (or a good sluicing with a hose)

I'll post again later when I remember the name of the stuff. . .

if you're absolutely wedded to a silicone compound, use one with aluminium or copper particles in it for better heat transfer. . . .

BUT:

I still stand by the 'Solder it' comment I made earlier.

the solder I'd use is Oatey's H-20 95 product; it's a zinc chloride based, tinning,  water rinseable product.  it WILL induce corrosion if left on your workpiece and exposed to water and air, so DO rinse it off THOUROGHLY when you're finished with your repairs.  

I have used this product with good results on structural brass and copper; I have also used it on brass models, and can report that it does wash off well, with no corrosion on un-painted brass after 1 year. If you're particularly worried, a vinegar pickling followed by a baking-soda rinse and a water rinse should cure any residual reactivity.

I am not connected with Oatey in any way.

whatever method you choose, good luck!

If you can't seperate the layers allowing safe torch use during a repair, I'm not sure of the best material to give a thermal bridge. Bonding may be a problem w/ all the heat cycles its exposed to, a combination of adhesive filler and mechanical clamps would be smart. Stainless steel automotive hose clamps come to mind.

I would use a product called "Muffler cement"

I used to sell it @ my last job, I can't get myself to recomend them yet! (&^$@!#%&*&&^%#@) that I worked for! Instead of screwing me out of X-mas Bonus
I got fired for stupid reason, new owner is a rich kid brat!
They didn't make the stuff but I did sell a lot of it, mostly thru the website I mantained for them.
If someone really wanted some I know where to buy it, (were they did, also a place I worked a lifetime ago)!
Best to do a web search for it.

PS: don't buy it from ppp!