As I'm Belfast based I can only answer from a UK perspective.
Like any modification you need to declare all to your insurers and the govt office dealing with road tax and MOTs, otherwise you could end up in a world of pain.
I bought mine converted. Previous owner had not declared it in the registration book (V5 here). Painless to do it but you may require an engineers report to certify that the conversion is safe. It didn't have any impact on road tax cost because of the age of the car.
At MOTs I don't mention the engine conversion. Its in the V5 for them to see. Ive taken it through 2 MOT's and it hasn't been mentioned once. Emissions wise its passed each time.
Cost to run would be expensive as an everyday car. But I use mine for only about 3k a year. Heavy footed driving is about 15 mpg, normal driving is 20-30 depending on mix of urban/rural/motorway driving. I have an N/A Rev 1 gearbox which has much shorter ratios than an equivalent Tubby box,. Makes for better acceleration but lower top speed and poor fuel economy.
Insurance can be expensive. I've paid on average £430 per year, fully comp, no claims limited to 5k miles and I'm 43 years old. Only two insurance companies in Northern Ireland will insure the car with this conversion.
Performance wise its somewhere between a 2 litre N/A and a standard Tubby. Its more of a cruiser than the high reving 2litre n/a. With loads of torque from 2k revs. I find the power deliver much more useable and more spread out than the tubby engine. Less of a wow factor without boost. Noise wise its incredible on full song.
With a short ratio box on a straight road shes marginaly slower than a standard Tubby to about 80,then the Tubby leaves it for dead. On twisties it doesn't matter what engine you have performance is much the same regardless of engine and is limited to how big your balls are and the suspension/brake set up