A similar method to what Don said is this.
Back in the day, I had to do a lot of attaching A-4B and C conversion noses to A-4E models. My problem was that the noses were White Metal. I tried attaching them with superglue, but it set up too quickly.
You can get more time by attaching a sheet of styrene to the back of your conversion nose, smaller than the ID of the fuselage part you have left after you cut it off to receive the new nose.(smaller because this is your wiggle room) Cut a very tight fitting sheet of plastic for up inside the fuselage, apply cement along the outside edge of your fitting sheet. Push this back into the spacing you need for attaching the nose and it's sheet,,,,,,using the nose and the sheet to push with. Take that nose piece back out of the hole. Let that new sheet dry in place.
You now have a large gluing area for attaching the nose and it's plug, and you can apply plastic cement to the inner sheet, put the nose up in place, and no cement will get around the outer diameter of your fuselage to nose joint. You will have plenty of working time to move that nose around to get it to line up the best. Stand the fuselage up so your nose doesn't slide while you let that cement dry.
Afterwards, you can just apply the smallest amount of brushed-on putty or surfacer to ensure a perfect appearing joint. There is no possiblity of an ugly gluing seam out there, because you never applied any glue out there in the first place.
Rex