I've just been through something similar, two of one of my cats three newborns died within the past 2 weeks, one of whom the vet said had a congenital heart problem and was the "runt of the litter", and another because cat flu got her and nothing we or the vet did could save her.
Both kittens were small, and dominated by the third surving kitten, the smallest who died first, I had to start on food early (which she took to with gutso) and give her milk because she was a third the size of the largest and had always been small, as well as having bad eye discharge which I had to keep treating but they always managed to close again.
So in a way, my situation, is similar to yours, but hopefully you can have a happy ending. The reason I'm bringing this up, is that I do have some advice, since I had to "intensive care" the two kittens I lost.
Be VERY careful force feeding the kitten. Aspiration is when milk or solids gets into their lungs, when they don't have proper suction or a cleft pallet making it difficult for them to suckle and when your force feeding and it's not ready to take it in and you do it too fast, it could get into the lungs and aspiration can be fatal to neonatal kittens - it can cause breathing difficulty and also a secondary infection.
Go very slowly with the feeding, a small tube syringe with some warm vet recommended milk should do the job, make sure you give the kitten plenty of time and patience. If in doubt get your vet to show you. Also get to them to show you how to check for dehydration. Dehydration will kill kittens mercilessly, it doesn't take long to take your baby, and it's the boogie man you have to keep an eye on.
Keep them in a sanitary place, the smaller they are, the weaker their immune system will probably be. Keep them warm, Mother cat should be keeping them warm, but if she's roaming, your kitten can't regulate its body temperature properly.
My cat used to roam a lot and the youngest diddn't have much energy to follow her, so we constantly had her up on the bed with us, laying against my partners warm, bare skin, and she loved it, snuggling up to him, finding just the right place where he was warm enough as his skin is pretty warm to the touch compared to mine.
A heating pad would keep your little one toasting, vets sell them, or a nice warm hot water bottle, warmed regularly to a temperature your vet suggests.
It's going to be hard if you go down the intensive care route, and I classify bottle assistance as intensive care, as it's full time, and also all the other stuff you'll have to keep an eye on, and you will stress, and at the end of the day hopefully your kitten will survive, but even with the hardest work, for instance I spent 8 weeks on eggshells only to loose 2 of them and fear for the last, and after spending $500+ on 4 days intensive care for my last whom I'm still currently grieving for and died last night.
However, it can all be worth it, even with the most negative story, if you follow your vets intensive care advice (small kittens are fragile and delicate), it could be a happy story like my older cats whom were small babies, and with care have grown up to be boistrous adult terrors lol.