Control line flying and the smell of burning castor, ahh what pleasant memories. Your .15 will fly your trainer but as Ed indicated it will possibly be marginal with very little if allowance for error. A good idea would be to take the OS .20 from the Hawk and put in your C/L model, sell the OS .15 (or possibly trade) and buy a good second hand .40 size engine to keep costs down for the Hawk.

Karol