Here is a relevant portion of the EULA for clarification on modification:
You may not Distribute any Licensed Technology except as follows:
a. Distribution to end users - You may Distribute Engine Code licensed to you under this Agreement (including as modified by you under the License) incorporated in object code format as an inseparable part of a Product to end users who are subject to an end user license agreement which explicitly disclaims any representations, warranties, conditions, and liabilities related to the Licensed Technology. However, the Product may not consist of or contain any Engine Tools nor provide functionality allowing the creation of standalone products utilizing the Licensed Technology; and
b. Distribution to other licensees - You may Distribute Engine Code licensed to you under this Agreement (including as modified by you under the License) in Source Code or object code format, or any Asset provided to you without additional charge by Epic, to an Engine Licensee who has rights under its license to the same Version of the Engine Code or Asset that you are Distributing.
You may not sublicense any Licensed Technology in Source Code format. You may not sublicense any Licensed Technology in object code format except to grant end users the ability to use, or to permit your publishers and distributors to market and Distribute, a Product that you Distribute as permitted in Section 1(a) above.
When you Distribute a Product to end users, you must provide Epic with advance notification at unrealengine.com/release, as early as reasonably possible, including the name of the Product, the format of distribution, unique Product id (where applicable), and the distribution channel(s).
You may not combine, Distribute, or otherwise use the Licensed Technology with any code or other content which is covered by a license that would directly or indirectly require that all or part of the Licensed Technology be governed under any terms other than those of this Agreement. Code or content under the following licenses, for example, are prohibited: GNU General Public License (GPL), Lesser GPL (LGPL) (unless you are merely dynamically linking a shared library), or Creative Commons Attribution-ShareAlike License. Code or content under the following licenses, for example, are allowed: BSD License, MIT License, Microsoft Public License, or Apache License.
You can go here https://www.unrealengine.com/eula for more information. Thank you and have a great day!