do you have an adjustable reamer? is your big end good  as i may have a new small end in my bits  chris  07901735443   swansea

take the crank case plug out  squirt some petrol in the spark plug hole   turn the engine over a few times to blow the crap out of the crankcase bung  hole, replace crank case plug  turn engine over with your finger in the spark plug hole  you will know what your compression is like  fit known good plug  gapped to   019  thou,   if a bantam has just been left in an out building for  more than 10 years the chances of  the crank seals detetioration goes up drastically  and the engine may start and run to  up to an hour  but on restaring from cold later the seals just give up the ghost through static hardnening over the long period of inativity,,,

i will try to precis this,,,d10 onwards one cam profile  on a taper can be set at any crank position some say 18 deg some say 30  deg BTDC,,,,, havnt played with one of these yet to get answers,,,excluding the early d1 lucas,,,,d1 to d7 the cam is held in position with a tiny  woodruff key, early wipac  magneto  had a 12 o clock keyway in relation to the crank pin, the cam looked  lumpy,,,later ones up to d7 had 2 types of cam and 2 diameters of magneto bowl wit a slight difference in advance of the magnet positioning,,,,all of these had a 11 oclock keyway position both d7 cams would work   but try setting the points to  .005 thou to see if it runs,,th long duration cam d7 was for the trickle charge small rectifier, nif you have  two much clearence on your spider to magnets the spark could be weaker  and have less output for your lights,  with magnetos,, no  plug suppressor cap!!

have you measured the mag plate bush diameter against a known crank thickness,,get a set of dividers or similar take a reading off the crank,,  get some small wooden dowel and reduce in a drill and sand paper so thy are the same dia,,,,there should be virtually nil radial play,,,  at 3/4 to full revs if there is radial play the end of the crank actalluy vibrates and interferes with the spark  and stutters the  engine,,,,or measure a new bush against the old one  fitted in the stator,,,,also  if it has an earlier  lumpy cam fitted  that limmeted the rpm a bit  compared to the smooth lift later cams from  d5 onwards,,,,,,,check the strength of the points  spring against another set  if its weak  this could also give the fault,,,  is it the long type of silencer,,,the short ones  dont seem to rev as high

you dont say what size ex system you have ,,is it wipac points,,,isit magneto or battery/coil or full electronic,,,is it the early or late d7 cam if it is wipac,,,, what air filter do you have,,,, are the fuel filters clear especially if you have the small spigot  push/pull tap,,,,,also if the mag surprt bush is worn it can flutter as you put it at higher revs,,,,,if its maneto try cleaning the points with wet and dry and some petrol soaked cloth and reset to  012 thou,,,

the concentric is more finicky  than the  monobloc,,and i have seen a porus casting,on one of these, check the cable top is relatively air tight,,i had a small runaway with an unkown monobloc,,turned out to be a missing venturi  block  seal washer,,,

as the saying goes,,grab it while you can,,,,