Easy Jafrh! There's no hard and fast rule here. It's pretty easy to build up a soft balsa cowl on the nose that doesn't have to be removed for engine maintenance. The obvious trick is to make the hole big enough for the engine to pass through. That means it will have to be at least as wide as the mounting beams. This can look very neat and professional if you nicely round the hole. If you want to go the removeable cowl route, you can make the opening fit a little tighter around the engine. I question whether that will look any better than a nicely finished permanent cowl on a model like this.

There was a feature article in R/C Modeler a couple of years ago on building up a cowl on a model like this. I think the article was based on a Tiger, but it was applicable to similar sport models like the V60. maybe someone else has the exact issue information...