Buy shorter chainring bolts.
Remove granny and big rings and use above bolts.
Choose a sprocket from cassette, pad out freewheel with old cassette spacers or piece of plastic drainage pipe.
Adjust rear mech limit screws to appropriate levels at act as tensioner.
Shorten chain if necessary.
Total cost <£5.
Instead of buying shorter chain ring bolts you can just use some thick washers instead or even chop an old chain ring up if it has had it.
If the limit screws on the rear mech won’t reach then use a short bit of old cable from the shifter end and the barrel adjuster to position.
Recommend buying a single speed chain ring though.