First of all, apologies----that is a 652cc engine in the Giardi, NOT a 5652cc engine (but I expect SOMEBODY would try and fit an engine of that size). At Radbourne Racing, a "Nardi" 652cc kit was comprised of a pair of 77mm barrels/piston units and a slightly wilder cam. In those days (the late 1960s) there were no other options, unlike today where there are a number of sources for these modifications. Basically, the engine is stripped, the barrel apertures in the crankcase are opened up (any competent machine shop should be able to do that job), the new barrels and pistons fitted and the engine then re-built.The increase in cubic capacity will instantly increase the Compression-ratio and the 'wilder' cam will enable you to use that extra capacity. I have put my thoughts on tuning the 500 engine down on paper, if you would like to contact me direct (
[email protected]) I will give you a copy of those thoughts. I can also give you details of Companies who can supply the parts that you will require.
The Giardy van that we had at Radbourne Racing was known to exceed 70mph on a number of occasions---even with 2 people in the van and a load of parts in the back!