645mm kristian... the 05 125 thumpy is a "Pitbike" sized bike... ie 12'' front, 10'' rear... and the frame is the pitbike size, not a midsize...
if you were to use 735mm forks, you would end up with a chopper... the easiest way to see what length fork you need, is to measure your forks from the centre of the axle, to the top of the forkleg. It would be somewhere around 630-660 mm... so your best bet, if you can afford it, is a set of Fast ace AS-01's or 02's, at 635mm length...
oh, and the I-shock you have on there, is actually a pretty good shock... or at least they were... if it's working well, keep it, if it's leaking, have it resealed... They were a great aftermarket shock "back in the day"....
oh, and going by your pics, it looks like you have a Jialing or Lifan start in neutral 125...