It looks like the dirt came from below more than above.
I have assembled quite a few engines in what I'd say is a dirty environment, dust from abrasive wheels all over ect
You don't need a clean room to do a good job, but you do need to clean the parts if they need it, you do need to keep them clean until they are assembled, and you do need to assemble it minimizing airborne contamination. This can be as simple as
Clean parts at the carwash and dry with compressed air, dishwasher.... whatever. Heat, water, and detergent is the key.
Cover all parts in trash bags when not working on it.
Blasting the parts and yourself with brake-clean is not going to work, unless the parts were never exposed to any harmful dust in the first place.....then is fine. Don't have to clean what isn't dirty so think ahead. Solvent does poorly at removing dust/grit because it's too light, you need water and detergent.
And just like painting a car......sweep and possibly wet the floor -before- taking off the trash bags, and don't blow air around or work in wind.
I use a propylene glycol mist to reduce static, and glue the dust down. Funny story how I discovered that. Once I did some part time mechanic work for a friend. He was working out of a warehouse, his brother was a carpet installer so he got all the scraps and put them down in his shop. Unfortunately, they all got soaked in anti-freeze. Crawling around in the slime carpet was.....memorable
, but I noticed there wasn't a spec of dust on anything in there.
Dust really varies far as what's actually in it, might be harmless biological material or might be highly abrasive silica dust....you don't know, but sometimes you find out the hard way.