Roll hoop windscreen attachment clips
 

My '99 doesn't have these clips, I'd like to install them. Are they avail as OEM? or?
Difficulty/ease of attachment?

My 97 didn't either but they are very easy to add. Parts are available easily, on mobile now so can't add a link but Google / ebay search '986 wind deflector mounting kit'. Just 3 small plastic bits and some pop rivets. Your car should have the holes under the rollbar inserts already, hardest part is getting these off (I used a pull bar wrapped in tape), then just rivet them in place. 10 minute job. There's a few writups and a video around also if you Google a bit.

The windscreen mounting kit is PN: 986 561 323 00

I recommend Sunset Imports, they tend to have the best prices. Should be about $25-30.