so many differing opinions on frames, they have all been modified some minor like rear sub frames welded or bolted on, most use the loops from the latter swing arm type's, in the 50 to60 the plunger was the most popular, its also a lot lighter, one uses rigid, and some use extensively modified rear single suspension units, disc brakes up front and home made tanks and seat units, but you could just bolt a few number plates on and race a std bantam, its certainly within the rules and allowed.
125cc Engines some say that sand cast cases are better and that D7 sand cast is the item most suitable for short strokes I agree with this, however the very early D1 Cases are thicker, and I use these!! so the choice is your to make, but one rule to follow, if your welding avoid the dye cast cases.
175cc welding of any kind other than a repair is not allowed, especially in the area of the transfers, if I where building a 175cc now, I would plumb for D10 dye cast as they are much stronger than the D14 and any of the sand cast types like the D7 above, especially around the bearings bosses even though there is perhaps less material. the main reason to avoid them if your short stroking as the welding weakens them.!
Barrel's:- !!!! I would plumb for the Brian white copy is cheaper !! available and can come ported and cylinder liner fitted by Brian who really does know what he is doing with Bantams, his are heavily modified copies of the original George Todd /Herman Mire cylinders done originally for the BSA six days trials works team.
carbs again so many to choose from 30mm to start with even for a 175. Mikuni or Amal in good condition is very simple to set up and lots of help as many use these.
ignition: Rex gaunt does a system fit straight onto your standard crank, but if your short stroking, possibly may want to shorten the shafts and I still run a 45 year old moto-plat on one engine, but there is also Ducati/interspan/boyar/Borat, and many others and Rex Gaunt Racing does a very nice system that will fit or he will fit it for you, he even does the back plate to bolt and allows adjustment, so again the choice is endless, prices are vary similar.
Like John said, sometimes requests for info like this can get missed, but never ignored, so keep trying I'm sure others have different ideas to my comments.
and will be along shortly to comment !.
regds Derek