Ask the place that you will be getting it from if it is a direct replacement or of there is any cutting or boring required. If it is a direct replacement then it should be about the same as removing the cylinder, piston and carb. If you have to fit the cylinder to the case (cuting and boring and stuff then I wouldn't get it because you couldnt go back to the original if you wanted)
Remember, if you are going to be using the original carb, you will prbably need to rejet the carb to a larger jet (if you need to, one might come with it) You may also have to get a newer exhaust although the one on the Avanti looks like it is a little better than the ones that come on other makes (expansion chamber and all)
Who are you getting it from?
Can they give any advice?
If it is someone like mopedwarehouse, they would probable be able to give you a good deal of support in what will be needed. I would make sure that you had someone to talk to at the place that you get it from.