Here's a magic trick for installing hinges on hoods or trunks for cars that already have the chassis/engines permanently attached. There needs to be enough space between the cowl and the back end of the hood to glue the ends of the hinges that stay with the body. The hood or trunk also needs to be flat from side to side. If not, see further down.
Using any of the methods shown online for making hinges, epoxy the hinge ends that stay with the body in place. When dry, this leaves the two "arms" that glue to the hood lying loose. These arms need some kind of flat surface attached to the top of each arm that will meet the hood's underside. Check fit before gluing, as the hinge arms may need to be bent upward to meet hood/trunk underside.
Next place a thin thread from side to side of the engine/trunk opening UNDER the two arms and long enough to clasp with each hand outside the opening. Load pads on each arm with 5 min epoxy, fit trunk/hood in place (over thread sticking out each side), tape hood/trunk securely in place, pull string taut and voila! Hinges glue blindly to hood/trunk.
If the hood has deep sides (like an older pickup truck) there needs to be small ring-like devices attached under the hood/trunk at the flattest area (a small bead on each side of the flat hood area would work) to make the string pull the arms up to the hood/trunk bottom. Care would need to be taken to place the body side hinge parts where they would meet the hood/trunk in this flat area. Thread the string through the "left eye," then under the arms, then through the other eye and out the side leaving enough loose string to be able to put glue on pads and finish as above. Just pull the strings.
The string method will need fine tuning for people who want hood/ trunk to stay up without support.
I hope this is useful.