Normally an engine is bored between .010" - .030" (thats 10 - 30 thousandths of an inch) basically 1/64" - 1/32" less than 1mm.
Do not take this the wrong way, but I would not recomend any engine modifications at this time. Drive the car the way it is, find out all of its good and bad points. Fix all of the mechanical, especially the brakes. Then when all is well, then work on modifying the engine, by then you will have much more knowledge of the car and know the questions you need to ask.
Also adding a "couple hundred" HP is not cheap.
You may want to read some of this post
Don't get ahead of yourself, take time to smell the roses, enjoy this hobby. It would be a shame to see a beautiful car like this wraped around a tree because you had a 500HP motor and no brakes.