The engine goes right in if you use carbs. You have to plug the injector holes in the head.

Other than that, and the gearbox, very little is interchangeable. To upgrade your front end, you need to find an 1800E or ES and swap the entire crossmember, and the rear axle as well, so the bolt circles match, and then you will have discs on all four wheels.

Unless it is truly rusted to death, or heavily crashed, I would think long and hard before parting out a 71 E. They are quite rare (approximately 2,500 sold in US) and the most desireable model of the entire 140 series. If not restorable for the road, the 71s make great race cars for the SCCA's IT series, where they are competitive with cars 25 years newer.