Open 'gate' of lugmount blanked of with wax, then shortened metal burr placed centally (blob of wax). Acrylic poured into lughole, filling void below - so creating retension. Acrylic then poured into mould and mould popped on top. No glue involved. Some kind of bond takes place between the acrylic and original plastic. Couldn't tell you exactly how - i aint no Chemist!!
Now considering not painting but adding jap's eye!
Might have some more pics of my old (your) KR. Me dads been sifting through his archive. Will post soon.
I've had good results using Plastex, easily available on the net. Did a half-hearted quick repair on a fairing mount on my Gag bike and it turned out very strong, if a bit ugly. But now I know it works, next time I'll take more care and time over it.