The most obvious 'under-the-bonnet' modification to Lazarus is an overflow recovery setup.  It came into being shortly after the very first trip that I made in Lazarus after I declared him ready to roll.  I polished him up, topped up the radiator and decided to motor on up to Abingdon Spares in Walpole, NH, about 100 miles away. When I got there I was appalled at the mess on the sides of the bonnet and the fenders.  It only took a few minutes to realize that the mess was anti-freeze that had come out of the end of the overflow pipe.  The bottom of the overflow pipe is in a low pressure area where the air is coming up through the engine compartment and out through the louvers in the bonnet.  The top of the overflow tube is just about at the filler neck and is right in line with the coolant that's being pumped up into the upper tank by the water pump.

I moaned about this to Skip Kelsey of Shadetree Motors.  I'm continually amazed at his wealth of knowledge of TD things.  Skip had been there before and had a solution - add a recovery tank.  The function of the tank is simply to prevent the coolant that normally comes out of the overflow pipe from getting lost (and splattering the sides of the bonnet and the fenders).  It's not an overflow system as on modern cars since the TD system is unpressurized.  In practice the solution is very simple.  I have an overflow tank from a late (77-80) MGB attached to my battery hold down bracket.  Tubing is then run from the bottom of the overflow pipe to the input of the tank.  The coolant that gets splashed (pumped?) into the overflow pipe finds its way into the tank and keeps things nice and neat.  I haven't lost a bit of coolant that way since I started using it.  Expansion from heat also causes some of the coolant to find its way into the tank.  Once things cool down the coolant is sucked back into the radiator. In the image below you should note that the pressure release lever on the radiator cap is in the 'open' position to avoid pressurizing the system.