[andylaurel]

I'll have a go here... seeing that no one else has...
男の子は魚釣りした魚を料理した。
Otoko no ko wa sakanatsuri shita sakana wo ryouri shita.
a)
Otoko no ko - boy
sakanatsuri suru (past tense is shita) - to catch a fish
sakana - fish
ryouri suru (past tense is shita) - to cook

I'm sorry, I don't know the verb 'to roast'

b) The simple past in Japanese is expressed by conjugating the verb into the 'ta' form. The two verbs in this sentence are a combination noun and verb. Sakanatsuri being the noun, suru being the verb - to do.
A normal verb such as 'eat', (taberu) can be conjugated into tabeta - ate.

c) I think the relative clause is added before the noun, with no joining particle. sakanatsuri shita sakana (the caught fish)
Someone might like to correct me here.

d)As for the boy the caught fish cooked. (notice that the pronoun "he" is not mentioned as Japanese don't seem to use pronouns all that often. "he" is implied)

Please note that the verb "to roast" has not been used, as I don't know it.