Door tops can be done over a weekend if the new tops have been painted and dried in advance. It's a little messy and fiddly, but once the first is complete, the others will be simpler.
The chassis might be OK with some welding - seldom expensive, and there are cheap Britpart replacement sections for all the complicated, rot-prone bits like rear x-member, outriggers and dumirons. If it's too far gone, a new chassis is expesive (£1200), and secondhand could be little better than your own (difficult to assess, as they rot from inside-out). A straight chasis swap can be domne in a week by the competent, but it's a lot of work and will need more than just the chassis.
The engine might just be suffering from the broken timing parts and bent valve push rods (usually bend sacrificially to prevent valve/piston damage). If you have avoided damaging the other parts, you'll just need a new chain and tensioner, headgasket and pushrods (8).