RE: Suggestions for my Inferno MP 7.5
A rebuild wouldn't go a miss, but the engine may have rusted up inside if your sibling never used any after run oil. The rest though should be fine as it stands, but I'd just strip it and replenish all the diffs and shocks. Those engines were pretty reliable and if it still has good pinch and shows no signs of rusty inernals then it'll be good to go.