In America one of the four Honda CB 750 preproduction has been sold for more than a tonne of euros.
That is of course an exceptional amount for an exceptional machine.
But putting savings in classic engines has quite a chance with the current prices on the NL market and the current interest rate.
Remember that nicely preserved, original specimens are the most interesting in terms of investment. Restored engines, no matter how well done, are usually worth less than those with a beautiful patina.
And something like a beautiful GS 550, or a Kawa KZ400 with only Church kilometers? They now cost twice nothing, look sweet and do not take up much space. In the meantime, the demand for all first types of Honda four-cylinder is growing, and a beautiful Yamaha 750 or 850 three-cylinder from Yamaha is also growing. But for a few grand, let's be three. are you still a buyer
Machines that have clearly lived, adapted or rebuilt, or machines that require 'work'? They are fun for enthusiasts. Not for investors.