imported_keys = wallet.storage.get( 'imported_keys' )
watch_only_addresses = { '1address1..' : '', '1address2': '', '1address3..' : '' }
imported_keys.update( watch_only_addresses )
wallet.storage.put( 'imported_keys', imported_keys )
Replace 1address1, 1address2 etc. with the most used addresses from your old wallet.
Then restart electrum and you'll find the watch only addresses listed under imported on the receive tab. Right click on each of them and select freeze so that electrum does not try to spend from these addresses (it can't anyway cause it doesn't have the priv keys)
The above is all for 1.9.8 and earlier wallets. If you are using the in-development version of electrum from the git repo then don't try this with that.