Willy has a 34mm wider front track courtesy of different offset 7J Speedline wheels (6.5J Speedline Vega on 16V Phase2) and longer 19 16V front lower wishbones. The Willy uses different shocks/springs to the 16V, though retains the braking system (apart from extra rear bias on the bias valve) and four torsion bars rear suspension. It also has a thicker front anti-roll bar.
The Willys bodywork is exactly the same as the Phase1/2 16V, apart from a slightly larger rear lip spoiler (on Phase2 models). Phase1 16v/Willy come with yellow fogs, Phase2 come with clear. Williams 1 is from Phase1, Willy 2 falls into both Phase1 and 2 and Willy 3 is Phase2. Willy 1 and 2 were not available with a sunroof or ABS - both of which the Willy had as options (I think).
The gear ratios are the same in1st, 2nd, 3rd and 4th as the 16V, although because the Willy engine cuts out at lower revs, they are in practice shorter. 5th gear is slightly longer. The Willy gbox has a tougher casing (courtesy of the 19 TD) and larger diameter secondary shaft, although the diff crownwheel is the same size as that of the 16V.
The interior uses re-trimmed 19 16V front seats (also used in 16V with leather option) and the Willy 1 has a numbered dash plaque. Willys also have blue boot and floor carpets, a suit carrier under the parcel shelf and specific steering wheel/gear knob.
The engine crank case is the same as the 16Vs, though the crank from the 1.9 diesel engine is used. 16V conrods are used, though the Willys pistons get thinner compression rings. It also has a mildly wider bore and shorter throw - giving extra torque at lower revs. The head has resin coating and wider inlet valves, along with polished inlet parts and a tubular manifold (cast on 16V).
To me, the Willy represents a more thorough makeover than the Cup is over the 172.