Hey Matthew
The A15 will need simple custom mounts, not a big job.
The Vanette 4/5 speed is quite different to the Sunny4/ 5 speed. Much shorter and I think the gear change linkage is above the box and probably wont fit in the thou tranny tunnel.

Vanette A15 is torquey at low revs but has a cam that does not like to rev over 4000 with zest
while the sunny version does
 
Best take the A15 vanette cam and swap it for a sunny A14 or A15 but they are hard to come by
so best just take the A15 vanette original cam to get it ground to GX specs.
 
There are other specs, but I found the extra 200-300cc on the A14/15 engines does this cam
justice as the A12gx needs 3500-4000 rpm before it starts to gain traction.
 
I had the GX cam profile in an A16 at first with bike carbs, headers, comp and alot of other work
but wished I kept the gx grind rather than go wilder as it became a pig to drive.
 
Beyond that would run a South African made Rowland inlet manifold for a 28/36mm carb,
10.5 comp, GX cam, nice headers plus 1 3/4 system and electronic ignition.
 
120y box or better is best but on a thouee, stock 1200 box can still do the job if compassionate.

Yep just bored to 77.5 for safety as A series bores are getting a fair bit of corrosion these days.
If I do an A16 twin cam, full sleeves are going in for double the price of just bigger bore job.
Mine will have 83mm sleeves for NA ca18de shaved pistons for flat tops giving 1775cc
Mazda twin cam head, 40mm CV carbs, headers, cams & Megajolt jnr ignition will scream