Thought I had lost this version of my answer--it sort of "just disappeared", the it "just re-appeared"!--very curious Alice
Sounds a good idea to me! However, you do not need to go the 'complete gearbox' route. The 126 bell-housing can be fitted to a 500 gear 'box'--you have to be careful when doing the change otherwise you have to re-do all the diff back-lash adjustment, but by being careful and keeping everything 'as was' (adjustment wise) you can get away with just changing them over. You will need a 126 starter--the 500 starter will not fit onto the 126 bell housing---different bolt arrangement, different position. One thing to remember--the 126 drive-shafts are too long to fit into a 500. You therefore HAVE to fit 500 drive-shafts, but if you have 500F, you will probably have the 25mm shafts already in the gear-box.
I have a "suggested specification" sheet for tuning up the 652cc engine--gives about 30/34bhp at the flywheel. If you would like a copy of it, e-mail me direct and I will patch it through to you--
[email protected] I am no expert, but my own engine has proved reliable, and surprisingly quick (for a 500 that is!). There are also a couple of pukka Abarth mods that are well worth carrying out when you tune the 500/126 engine--those mods are included in the 'bumph' sheet.
Another thing to remember is that if you make the beast go faster, you will have to look at uprating the brakes. To a degree the ordinary 500 brakes are fine, but if you start using the extra performance, they become marginal. Is it a complete 652cc engine that you have? or just a bare crankcase? Whatever you do, all the best, keep us informed.