Andy
The Funtana 40 needs a bigger engine than the 40 2s or 70 4s suggested to get the best performance the 90 you propose is probably over kill but you would have plenty of power in reserve, I suggest you mount your engine to some suitable bearers preferable T shaped so that you can get the engine as far back as possible turn the airframe vertical place the engine and bearers on the bulkhead, put the cowl on in its final position, centralise the engine with the front opening i.e. spinner back plate, take off the cowl and mark where the bearer mounting holes are on the bulkhead, I'm sure you can do the rest, I had to attach spruce cheacks to the bulkhead to move the cowl forward so that I did not have a big gap between the cowl and the spinner.
Incidently with a 70 in mine I had to mount the servos under the cockpit to balance without additonal weight, you may not have this problem with a 90 however, weight is to be avoided at all costs on a 3D maching, if you do have a c of g problem move the servos and close loop the linkages for positive input.
Hope this helps.
Mike