Often have we heard the contemplations of egg size, fruit sweetness and cheese names in history in search for either the real thing or modern counterparts.
I'm not going to get into a whole lot of details here as there really isn't the space or time for it, but will break this up into three simple points.
1. Eggs: Chickens were selected for egg production and meat, this means how long a chicken can produce and how soon a young chicken can get up to size. This did not specify how big of an egg a chicken will lay, in fact a chickens eggs will vary within it's life cycle so you will end up with varying egg size, and quality, from the same chicken. If you're really concerned about getting the right amount off egg when your book says 3 eggs, but does not specify egg further, then don't worry and just use 3 eggs BUT be sure to use standard chicken eggs at least (unless specified otherwise). (by standard, I mean not Bantam eggs which are from miniature breeds)
2. Fruit: If you can find a heritage fruit to suit the needs of your period cookery, then by all means use that fruit, but if not... then all we can do is approximate from description. One thing we should try to not do is generalize... such as all older types of oranges were bitter (there are 16th century mentions of sweet oranges, for example).
3. Cheese: Just because it is mentioned by name in historic cookbooks does not mean that what we eat today is the same cheese. The processes may be very different (I know Cheddar has sparked a lot of talk) and even small changes from milk production up to curing would make a difference in the final outcome. While there isn't a whole lot we can do about some things, we should at least accept that there are differences between the early and modern versions of many of these products.