What you have is nothing more than a PARTS TRACTOR that has a value of about $300.00 on the open market. The Onan is far more expensive to rebuild than the Kohler used in the 444 is and anyone considering buying your tractor is faced with spending upwards of $2,000.00 for a thorough rebuild. The only way that such an expenditure is remotely justified is if you intend to keep the tractor for the next 10 years at minimum. At an average annual use of 50 hours, the engine would only have 500 hours on it after 10 years and that's a mere 25% of the lifespan of a properly serviced Onan.
Sorry about your bad luck with this one but if the above valuation isn't to your liking, then perhaps you should keep your eyes open on Craigslist and e-Bay for a good, used Onan at a price you can live with.