hmm yeah the AGB-21 is a really dated design... not the best pit bike on the market, put it that way...
however, depending on what you want to use it for, it will be fine for just putting around...
a lifan 150 will fit in that frame pretty easily, however you will reach the limits of your suspension very quickly...
A fast ace shock from DHZ will improve the feel of the rear suspension, but there isnt much that can be done with the forks, short of either replacing the front end altogether, or changing the oil in the forks..
having said all that, i bought one of these around mid 2004, and the engine is still going strong... i reckon that's a ducar 125 motor... not the best performer out there, but they are reliable...
The carby on your new bike is going to be your biggest problem, putting it bluntly, they're terrible... your first stop would be to replace it with an OKO 26mm flatslide.... DHZ also sell these as a kit on ebay, and there are a couple of threads on here in the Tech section devoted to running and tuning OKO carbies...
i wouldnt worry too much, it might not be the greatest bike on the market, but with a bit of work and proper set up, you will get a decent life out of it.