Well it seems that you have very strong regulations in Spain.
Here in my country you can do whatever you want with your car. There are few modifications, for which you have to pay only once to be approved by police, like some fancy paint job, and some engine modifications, like NOS , but this taxes are funny....around 30 euro.
We even don't have engine numbers written in cars documents. I can put engine from some truck for example and there wouldn't be any problem if i can pass Car Technical Inspection every year, which check only brakes, lights and emissions.
So, i'm prepared that i'll have some major problems with this, but i will try. I have some ideas, so if i come to success i will post it here. Otherwise i'm waiting any suggestions

Thanks.
Aut inveniam viam, aut faciam...