Hi Ignas.
Welcome to the forum. Wow!! You have some rot there. It is very extensive and I would think only the bravest would consider repairing the chassis. However it is possible. There are many chassis pieces available, from Der Franzose or Melun Retro Passion.
I think you have two options. Either take a holiday to the South of France with a car trailer or light truck that will carry a scrapped R4, and bring one back with you.
Or if you are contemplating spending money on the existing chassis, look to see what chassis parts you require, and their costs before you commence. (A drive to France and back may be much cheaper, and will save you much time.) You will have to sand blast it first to see if you have any steel good enough to attach something to. You will then have to construct a heavy steel jig to carry the existing chassis. This you bolt the chassis to, so it does bend when you weld it. I think Malcolm has posted the important dimensions that are essential for putting everything back on in the correct way. If not I am sure we can find them for you, as I have a body work manual with them in. Then you have to cut off all the rotten pieces and then spot weld the new parts on. It is a lot of work. Malcolm has done a very good write up on chassis repair.
I personally would take a trip to France. Whichever way you go once up and running you will have a vehicle with such character and individuality that the effort will have been worth it, just keep an eye on the expense.
Best of luck and please continue to use the forum for help. There are many great people out there always willing to offer help and advice.
