The closest you're going to get is the clearJournal command, which will eliminate all of the financial information, including your bank balance. That last is easy enough to fix if you have the sandbox mod installed.
You would still be stuck with the flat line from 1850 to present, but at least the changes from that point on would be shown on a reasonable scale.
For the future, there are a couple of options:
1. ensure the sandbox mod is activated. Then pause the game and make a note of your current cash balance.
Do your landscaping and then click directly on your cash balance. That will open up a dialog where you can enter your previous cash balance.
2. there is a mod in the workshop that makes terraforming free. You either leave it activated for the whole game, or, if that felt like cheating, you could activate it only for the type of map alterations you described.

Oh thanks man. I should have known earlier to not use the no cost mod.

However I tried the clearJournal function but I was not successful

Here's the trace:
>> game.interface.clearJournal()
error: [string "game.interface.clearJournal()"]:1: attempt to call field 'clearJournal' (a nil value)
stack traceback:
=[C](-1): clearJournal
game.interface.clearJournal()(1): ?

If you have debug mode enabled, you could run it from the console. Otherwise you'll need to run it from a game script.

Is there a way how to modify via script journal data itself?

I would like to use game.interface.book or new API make.bookJournalEntry but I havent found any documentation how to use it .

Have a look at this. I created it to clear out the financial history generated during the creation of a map:
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=2075070872

Yeah that was the mod I looked in it yesterday. I saw you were using the game.interface.book function. Do you know how to use it to book records in different category than "other". I would like to recreate my journal as I have all the data.

Alternative would be this make.book.JournalEntry API which I do not know as well.

P.S. clearJournal works from script as expected.
Thanks again.

Just for the other I have found the old API documentation
http://transportfever.com/wiki/script-doc/index.html

new API is here
https://transportfever2.com/wiki/api/index.html