I guess it is because they give you a 12 month/12,000 mile warranty when buying the turnkey from them. Years ago Kirk would give you a 2K credit if you didn't get the engine from them; it seems that things have changed. Since that's the case your best bet would be to just get the basic 1600 cc engine with your order and then tear it down and build it up after the warranty period expires. You'll not only save money now but you'll be able to at least use the case, heads, fan, alternator & sheetmetal. When upgrading change all the internals, pistons and cylinders, get your existing heads ported, get better carburetion, ignition and exhaust and you'll be set.