Let's go into further detail: Yes, you can increase the displacement of the engine up to the limit in your class -- but the outside appearance of the engine must remain the same as when you took delivery from the factory. If you have a picky inspector or are protested -- you might not even get away with an engine of a different color - unless you can prove that that different color was also a factory available thing (like if you paint the block and heads bright green once you're done with the boring bar).
At ECTA events you can run up in engine class in Production -- that is, your 1,000-cc maximum engine would be allowed to run against Production records in classes of 1350, 1650, 2000, 3000, and 3001+. At SCTA events you may only run in the class the engine - as built and supplied by the manufacturer - naturally falls into.
Where are you? Your moniker notes don't tell us -- maybe there's someone near enough that you could swap stories and help without going through the Forum (not that there's anything wrong with that, though).