Paul does the 5 speed you have need work? whats wrong with it?
probably best have the other box checked to see if it needs rebuilding.
 
Also Thailand has some good machining shops very old school but
japanase lathes and other japanese stuff why dont you rebuild an A15
with bigger bores and keep your A12 until the A15 is ready?
 
You could put the new 5 speed in for the meantime just till the A15 is
ready as the A14 is an upgrade but depends on the head is has etc.
For the price of things there and whats available new in parts you can
build my dream engine that Im still trying to do here as money comes in.
 
eg. 1775cc A15
A15 used stock prepped rods (cheap to do there)
use 83mm sleeves (cheap to get there)
ca18de or det second hand pistons (cheap to get anywhere)
if using DE pistons need top bulge machined off (easy to do)
this will give you a flat top piston
New Chrome rings (japanese brand or american)
R1 bikes custom manifold (cheap from ebay & Thai can make)
Headers you have or can get made (cheap to make there)
Camshaft regrind for more duration (cheap)
H89 head ported (stock vavles, new springs for 7500rpm)
 
Clutch use L16 clutch with stock A10 flywheel machined
to fit the L series Nissan clutch of 200mm. A10 flywheel
on original engine is the lightest flywheel of all A series.
Have the rods, crank, flywheel, front pulley and clutch
all dynamically balanced (use A14 single pulley its the lightest)
 
Shortened diff to suit (cheap to make there)
eg. LSD vanette, Largo or other.
 
You will have depending on the camshaft about 120-130hp engine.
Similar to a mild Nissan l18 but better torque and revvs due to
lighter crank, fly, valves, smaller bearings surfaces & lighter pistons.
 
I wouldnt think twice with the prices for labour you have there as the
parts required are very little with most expensive been the bike carbs
at 150usd and the ductile iron sleeves at 30 dollars each  x 4 $120.