Yeah, changing the beam angle on one of those would require finding a lens that is the correct size and shape to do what you need it to do and still be able to focus. If you have a lot of spare parts from other lights laying around I'd say go for it and see what happens. I'm sure it'd at least be a nice learning experience even if you don't find a working solution.
Lightweight is a relatively vague term. For me, lightweight is anything under 40lbs considering many other fixtures I use are 80+. It seems the lightweight in this market is in the 20 pound range at most. Having replaceable gobos is a really nice feature. Making a gobo wheel from scratch can be expensive and time consuming, plus you need to make sure its 'stops' are set in place correctly so the unit calibrates properly.
If you're still considering fixtures (to buy), I'd really suggest you stretch your budget a little and buy something that fits your needs out of the box. Not that I'm against engineering solutions to save money, but in this industry you really want things that work correctly and reliably. Being an American DJ forum I won't recommend equipment from other suppliers and American DJ / Elation have some nice choices for affordable moving lights.
Seeing that the X-move is a candidate for you, you might want to take a look at the VIZI LED Spot or the VIZI Spot 5R. Both units have the replaceable gobos that you're looking for and are still pretty light weight. They have a 14 and 18 degree beam angle respectively, so the VIZI Spot 5R will be nice and wide, especially for a 'spot' unit. Both are decently affordable too. If you take the price of an X-move plus the cost of a custom gobo wheel, you might be near the price of one of these units anyways, and you'll get *much* better performance out of either one of those. Plus you get a few more gizmos to play with too.
You could go a bit further and look at the Elation Design Spot series. Even the DS250P has all the toys you want plus an interchangeable beam angle. Step up to some of the higher models and you'll get a zoom feature. Now these units cost several times more than what I think you're looking to spend, but it's still fun to look and consider.
I hope this helps, and my apologizes if this isn't the route you're looking to go. From my experience though it's really best to spend a few extra dollars now and get what you need versus trying to save a few buck but have constant problems with the unit not doing what you need it to do. Years from now you won't remember that you spent a few hundred extra for your lights but you'll remember how nicely they worked for you.