like for any engine change in any car,,,,,,,,,,,,,
If you change the engine after a car has been registered you have to inform DVLA of the new engine number as this then confirms if you stay in the same "road tax " class..... i,ll explain.....
hypothetical scenario....
if you had a car on the road that lets say had a 1300 cc engine & you change it to a 2.0ltr zetec then you must inform the dvla to ensure their records are kept up to date & you would be then charged the appropriate road tax for your new engine capacity,
build your car, enjoy it for a while ( 6 to 12 months), get used to driving a "7" style car , then think about upgrading your engine,
cheers
andy