TBH your not going to get allot of help here, Every month new members ask the same question search
(bike engines) and read the threads. ever since this forum has been online I haven't heard of or seen a road legal UK registered bike engine'd 126 conversion, Once you start cutting the body to get the mechanical's in, the car is subject to an
[IVA Test] once you get past that stage insurance is not a problem as long as you have deep pockets every modification has to be declared, engine, brakes, wheels, tires, seats and any other mods you might have done otherwise you will be screwed if you have an accident or someone runs into you. My advice is unless you are a qualified mechanical engineer I wouldn't attempt the conversion, you only have to look on Ebay and see how many 126 Projects are for sale every year, because they start off with good intentions but then realize they are out of there depth.
Sorry but I'm just telling it how it is, maybe you can prove us wrong and if so Good Luck!
Checkout this thread
[Another bike engined project bites the dust]