Questions about replacing a septic drain-field

After pumping out our septic tank, I'm told that my drain-field is clogged, probably with roots. After digging out the entrance to the drain-field, I see that there is no distribution box, just Teed off PVC branching into the drain lines.
I can't afford $5k+ for professionals to replace my field, but I don't want to do a crappy job. There's some good information here , but can I ask a few questions:
1) Is the lack of a distribution box a big or little problem? Can I just copy what was done 30 years ago?
2) If I just buy 4" schedule 40 PVC and drill 1.2"-3/4" weep holes, how far apart should they be? Or should I buy something else?
3) What kind of soil barrier should I put over the gravel above the trenches? Should I use something quality like tar roofing paper?
I live in the Florida Keys, where the soil is about 6" deep over petrified coral and limestone. I plan on manually excavating one trench at a time, as time allows, and replacing the old with the new, scrapping the old next to or on top of the new before filling it back in.
Thank you for your help.
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