For migration from magento 1 to magento 2 you have to migrate or convert following details
c) Customization you did
For more information on this please follow.
1) Export and Import your Customers, Orders and Product Details.
2) Use any comparison tools like Beyond Compare to check if is there any core files modification exist. if exist make a note of it.
3) Compare the third party extension if you have modified it or not and make a list of it.
4) Check whether you have directly created database tables (Without using Extension script). if exist make a note of it.
5) Check Availability of Third party extension available for magento 2. if you not exist you have to convert it yourself.
6) Install Magento 2 Latest version and Import the data
7) Install third party extension available for magento 2, which you have already installed in magento 1. Modify it according to the result of Step 3.
8) Build Extension by yourself which are not available or you have build for your own purpose.
9) Write Custom Plugin for the report you get from step 2 and a plugin for orphan database tables.
Hope you understand the concern.