-TRX300 Won’t start-ready to give up

I posted this in the new member section- I thought I should try here.

Hi all
I just joined this site. My name is Teresa (from Maine) My son who is 16 has a 93 Honda 4 trx 300. My husband died last year (he could fix anything) and we don’t know what to do next.
This machine will not start. I had a friend replace the following parts-

Battery
Ignition
pulse generator
starter
coil/alternator
cdi box
I assume he replaced/checked the fuses

every time he said" this is the last thing it could be". Well, I have over $400++ of new parts and it still won’t start. I can’t keep dumping money in this thing and can’t even sell if it s not running.
What should I do next?? Neither my son or I know that much about this and don’t have a lot of money to spend. Barring a new wiring harness what else could we possibly replace
Thanks