Just my two cents worth. Does it really weigh 9lbs.? If that is what it will weigh in the model then I say it's too heavy. Most if not all of the 40-50cc engines being used are around 4 1/2 lbs. I believe a chainsaw engine is better than a weedie. Having the magneto on the back shouldn't be a problem if it fits in your model. You might want to consider doing a gas/glow conversion also which eliminates both magneto and EI and will make that engine a lot lighter but I don't think it will be light enough. I have been told the "Rule of thumb" is 1hp for every 10lbs. so you have already used 1hp and that means your model should only be 10lbs. without the engine. Good luck.