Hello Mitzi, I am doing the complete opposite, a Supa 5 into es250 chassis. I believe the top end will go on, have a look at
www.starfield181.co.uk and read his excellent diary of the Trophy sidecar project. He has built (send him an email for his advice, I have emailed him in the past and had a friendly response) an etz250 with the spiked (TS250) head onto it (Just remember that you can not use a TDC gauge to check ignition timing with the off centre spark plug hole, like you can with the central spark plug position, but don't let that put you off if you go for it).