You could use a pan adaptor, but that would look terrible, but it would not leak. A better way would be to use a universal rubber adaptor with jubilee clips. But I am an electrician, a plumber would easily identify a suitable solution, mortar will not do the job as the coupler on the clay pipe is broken.
Because the flange is broken on the top of the clay pipe I would create a small wooden box to give in effect a shuttering around the pipe, stuff some horsehair (yes I've still got some!) or plastic into the bottom of the box to prevent mortar falling through into the clay pipe, then finish off with a 3 to 1 sharp sand to cement mortar mix, trowelled off so you end up with a small square of mortar say 250 by 250 by 150 angled down to look more acceptable.., Alternatively you could fit a cast iron to clay adaptor which would mean pushing the adaptor inside the clay pipe and getting the other end over the cast iron. If flexible enough you might manage it, unlikely, or cut the cast iron and fit a longer adaptor with jubilee clips. Still need to finish off with a bit of mortar.
Mortar or prompt is the correct way to do it the other two are bodges. Put a piece of hemp rope round the cast to centre up and then flaunch with mortar or prompt as stated by Abrickie and nigel.
The only place they should be used is for connecting the radiators in vehicles, they are, and also look a total bodge when used for plumbing and drain work.