Hi @Namechangegardens,
A bit late to the thread here sorry! Congratulations on your little boy, I hope you've been enjoying your pregnancy so far.
We also had a cleft lip (and palate) diagnosis at the 20 week scan of my little girl. I'm now 30 weeks along and she looks totally healthy, same as your son, and I had the exact same concerns that you did too.
I'd definitely advise that you have a really good read through the CLAPA website and speak to a parent peer supporter which is what we did. You may have been put in touch with your Cleft Nurse Specialist by now too - don't be afraid to ask them any questions at all - particularly around feeding as I was very worried about this and our nurse really put my mind at rest. I sent our nurse a monster list of questions and she addressed every one. Have a read up about colostrum harvesting as this might settle your mind about very early days feeding before you find what bottles/methods will work best in the event that you can't breastfeed (if you did want to try breastfeeding that is - I don't want to make any assumptions!)
We were invited to take part in a new study for parents who've had an antenatal cleft diagnosis called The Cleft Collective Birth Cohort Study. We figured our baby will be afforded such great care because of past research and decided that if we could give something back and assist with improving care even more, then why not! That helped us to feel involved and more positive. I'm sure you could ask your Cleft Nurse about this if you haven't heard about it already.
If you use Instagram, I started following other cleft positivity accounts and parents of babies with clefts so that you can see how very real people go through it all. That helped me LOADS because I realised that the other parents were just typical, normal people like us and celebrating the lives of their gorgeous babies just like anyone else. It helped me to understand what a normal thing this is to be born with now. Way more common than I realised. On the spectrum of things parents can be told at a 20 week scan, we eventually realised we'd been lucky. I don't say that to belittle your worries at all - of course we all hope for a "normal" 20 week scan which you and I didn't have. But this is so, so manageable and almost everyone I've spoken to knows someone with a cleft; a friend/friend's child/went to school with a child who had a cleft etc. And they all said "you wouldn't even know it if you hadn't already been told".
I found that mentally framing the news in a different way helped me to digest it better too. I very much went through a difficult "why us" period, but tried to reframe it as "there was a 1 in 700 chance this could happen, I'm so lucky that she chose us". Your son's cleft developed and has been this way since he was probably about 5/6 weeks in the womb; he's the same baby you've known in there all along and no doubt you've loved every second of him so far 
Once I'd got over the shock of the news initially, I was overwhelmed with love and already felt proud even though I knew my experience of motherhood would be slightly different to what I'd envisioned. And it is totally OK for you to grieve the experience you know you may not have now - please allow yourself this.
In all honesty, I am still struggling to accept that I almost certainly won't be able to breastfeed so if you are struggling with this too; I understand your heartache. Just remind yourself that there are X number of reasons why breastfeeding might not have worked out for you and baby totally unrelated to clefts. Same with concerns about scarring, speech or hearing development problems (which are only ever a maybe, by the way) - anybody's baby can be born with these and need some extra help or have a nasty bump and need a few stitches any time. The difference is we cleft parents just get an extra heads' up about it 
When it comes to peoples' reactions, I haven't got there yet but I do share your concern! People probably will stare, yes. I'm already prepping a list of responses; "yes she's gorgeous isn't she!", "I can't stop staring at her either!" or anything considerably less friendly if people are unkind. Time to grow a thick skin and accept that we can't control peoples' reactions. I wouldn't worry about giving a curt response to anyone who was unkind about any baby though! They'd deserve what they got.
Anyway, sorry for the essay but I wanted to let you know what has helped me feel positive and extend that to you. You're going to be a wonderful mum and who knows - maybe we'll find ourselves meeting at a CLAPA event or something in future!! Feel free to DM if you would like to chat more about anything x