How do you mean quiet?
Its a 20 year old derv engine at its core but new engine mounts, using the engine cover, soundproofed bonnet and adding some soundproofing to the bulkhead behind the engine all make a big difference to engine sound
If its general NVH then keeping then silencers in place, some dynamat/fatmat/silentcoat on the doors, bulkhead, floors and boot all make a big difference. More so with some closed cell foam added (not open as it traps water and causes rot)
Plus making sure all nuts, bolts and tight, bushes are in good order etc..
I recently drove a stock 110 back from Stoke to South London, tbf didnt feel much of a difference in terms of NVH to my D2 or my GT86.
Regularly drove my old 110 everywhere, including 3 trips to Barcelona but that was heavily soundproofed and comfort improved to a higher standard than a stock D2 though